Inclusion Criteria:

1. * Able to understand and give written informed consent.
2. * Assigned female or male at birth, 18 years of age or older.
3. * Eastern Cooperative Oncology Group (ECOG) performance status 0 to 1.
4. * Measurable disease by Response Evaluation Criteria in Solid Tumors (RECIST) v1.1 criteria by investigator assessment.
5. * Organ function requirements:

1. * Adequate hematologic function
2. * Adequate hepatic function
3. * Creatinine clearance
4. * Coagulation
5. * Tissue requirement:

* Parts A, B, C, and D:

1. * Pre-treatment tumor tissue is required.
2. * Parts A and C backfill biopsy cohorts:

1. * Participants must agree to fresh pre- and on-treatment biopsies.
2. * Participants assigned male at birth and participants assigned female at birth and of childbearing potential who engage in heterosexual intercourse must agree to use protocol-specified method(s) of contraception
3. * Willing and able to comply with the requirements and restrictions in this protocol

* Part A (GS-0201 Monotherapy Dose Escalation) Inclusion Criteria:

1. * Histologically/cytologically confirmed progressive/advanced solid tumors with selected molecular lesions.
2. * Participants must have received, been intolerant to, or been ineligible for all treatment known to confer clinical benefit or have a contraindication to receive the therapy

* Part B (Dose Expansion) Inclusion Criteria:

1. * Disease documented as:
2. * Cohort B1:
1. * Histologically or cytologically confirmed progressive/advanced selected solid tumor diagnoses harboring defined molecular lesions
2. * Participants may potentially be required to forgo treatment with approved agent(s) to be able to participate in the study
3. * Cohort B2:
1. * Histologically or cytologically confirmed progressive/advanced solid tumor diagnoses harboring defined molecular lesions not included in Cohort B1
2. * Participants must have received, been intolerant to, or been ineligible for all treatment known to confer clinical benefit or have a contraindication to receive the therapy

* Part C (Dose Escalation) Inclusion Criteria:

1. * Histologically or cytologically confirmed unresectable locally advanced/metastatic selected solid tumors

* Part D (Dose Expansion) Inclusion Criteria:

1. * Disease documented as:
2. * Cohort D1:
1. * Histologically or cytologically confirmed unresectable locally advanced or metastatic triple-negative breast cancer (mTNBC)
2. * Cohort D2:

* Histologically or cytologically confirmed unresectable locally advanced or metastatic HR+/HER2- (IHC 0, IHC 1+ or IHC 2+/ in situ hybridization (ISH-)) breast cancer.

Exclusion Criteria:

1. * Pregnant or lactating females
2. * Known hypersensitivity to any of the study drugs, its metabolites, or formulation excipients
3. * Requirement for ongoing therapy with or use of any prohibited medications described in the protocol
4. * Participants with myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML) or with findings suggestive of MDS/AML
5. * Refractory nausea and vomiting, chronic gastrointestinal diseases, inability to swallow the formulated product or previous significant bowel resection that would preclude adequate absorption of GS-0201
6. * The therapies listed below within the specified timeframe:

1. * Major surgery (excluding minor procedures, eg, placement of vascular access, gastrointestinal/biliary stent, biopsy) \< 4 weeks prior to planned Cycle 1 Day 1
2. * Immunotherapy or biologic therapy \< 21 days prior to planned Cycle 1 Day 1
3. * Chemotherapy \< 14 days prior to planned Cycle 1 Day 1, or \< 42 days for mitomycin or nitrosoureas
4. * Targeted small molecule therapy \< 14 days prior to planned Cycle 1 Day 1
5. * Receipt of experimental therapy within 21 days or 5 experimental treatment half-lives (whichever is longer) prior to planned Cycle 1 Day 1
6. * Hormonal or other adjunctive therapy for cancers other than the cancer under evaluation in this study that started \< 14 days prior to planned Cycle 1 Day 1 are not permitted. Hormonal therapy, bisphosphonates, somatostatin analogues, and leuprolide are permitted if started ≥ 14 days prior to planned Cycle 1 Day 1
7. * Radiotherapy within 2 weeks prior to planned Cycle 1 Day 1 and the radiation is not administered to a target lesion
8. * Any prior allogeneic tissue/solid organ transplantation, including allogeneic hematopoietic stem cell transplantation. Participants with a history of autologous hematopoietic stem cell transplantation are also excluded
9. * Have not recovered (ie, Grade 1 or lower) from AEs due to a previously administered agent
10. * Prior treatment with approved or experimental prohibited agents as detailed in the protocol.
11. * Diagnosis of immunodeficiency, either primary or acquired, or requires systemic corticosteroids (\> 10 mg of prednisone daily, or equivalent). However, replacement doses, topical, ophthalmologic, and inhalational steroids are permitted
12. * Have an active second malignancy
13. * Have known active central nervous system (CNS) metastases
14. * Participants with carcinomatous meningitis or primary CNS tumors are excluded regardless of clinical stability
15. * Meet any of the following criteria for cardiac disease:

1. * Myocardial infarction or unstable angina pectoris within 6 months of enrollment
2. * History of serious ventricular arrhythmia (ie, ventricular tachycardia or ventricular fibrillation), high-grade atrioventricular block, or other cardiac arrhythmias requiring antiarrhythmic medications (except for atrial fibrillation that is well controlled with antiarrhythmic medication)
3. * QT interval \> 470 msec
4. * New York Heart Association Class III or greater congestive heart failure or known left ventricular ejection fraction less than 40%
5. * Meet any of the following infectious criteria:

1. * Have active serious infection requiring antimicrobials
2. * Have active hepatitis B virus (HBV) or hepatitis C virus (HCV), or HIV. In participants with a history of HBV or HCV, participants with detectable viral loads will be excluded
3. * Participants who test positive for hepatitis B surface antigen. Participants who test positive for hepatitis B core antibody are eligible with a negative HBV DNA by quantitative Polymerase chain reaction (PCR)
4. * Participants who test positive for HCV antibody. Participants who test positive for HCV antibody are eligible with a negative HCV RNA by quantitative PCR
5. * Participants who test positive for HIV antibody
6. * History of pneumonitis requiring treatment with corticosteroids, interstitial lung disease, or radiation pneumonitis requiring steroids
7. * Symptomatic ascites or pleural effusion
8. * Have other concurrent medical or psychiatric conditions that, in the investigator's opinion, may be likely to confound study interpretation or prevent completion of study procedures and follow-up examinations
9. * Any medical condition that, in the investigator's or sponsor's opinion, poses an undue risk to the participant's participation in the study
10. * Use of any live vaccines against infectious diseases within 4 weeks (28 days) of initiation of study drug(s) (inactivated, viral vector vaccines, and messenger RNA (mRNA) vaccines are allowed; seasonal vaccines should be up to date prior to planned Cycle 1 Day 1)
11. * Parts C (Dose Escalation) and D (Dose Expansion): Combination Cohorts:

1. * Participants with active chronic inflammatory bowel disease (ulcerative colitis, Crohn disease) and participants with a history of bowel obstruction or gastrointestinal perforation within 6 months prior to planned Cycle 1 Day 1
2. * Participants who previously received topoisomerase 1 inhibitors or antibody-drug conjugates containing a topoisomerase 1 inhibitor
3. * Known severe intolerance or life-threatening hypersensitivity reactions to humanized monoclonal antibodies or intravenous (IV) immunoglobulin preparations; any history of anaphylaxis; history of human anti-human antibody response