The goal of this clinical study is to learn more about the study drug GS-1206, including its safety, tolerability, and antitumor activity in adult participants with solid tumors.

Key Inclusion Criteria:

• All individuals must have a tumor with protocol-specified mutation as determined by local health-authority approved test.
• For Part A (including backfill cohorts): Individuals with histologically confirmed advanced or metastatic solid tumors who have progressed following treatment in the advanced or metastatic setting known to confer clinical benefit, unless the individual refused, is intolerant to, or is not eligible for standard-of-care (SOC) treatment.
• For Part B dose expansion, individuals with protocol-specified tumor types will be enrolled.

Key Exclusion Criteria:

• Use of any of the therapies listed below within the specified time frames:

  1. Investigational drugs (drugs not marketed for any indication) within 28 days prior to Cycle 1 Day 1.
  2. Anticancer biologic agent or immunotherapy within 28 days prior to Cycle 1 Day 1.
  3. Anticancer chemotherapy or targeted approved small molecule therapy within 21 days prior to Cycle 1 Day 1, or 42 days for nitrosoureas or mitomycin.
  4. Hormonal or other adjunctive therapy for cancers other than the cancer under evaluation in this study that started within 14 days prior to planned Cycle 1 Day 1 are not permitted. Exceptions: hormonal therapy, bisphosphonates, somatostatin analogues, and leuprolide are permitted if started at least 14 days prior to Cycle 1 Day 1.
  5. Major surgery (excluding minor procedures, eg, placement of vascular access, gastrointestinal/biliary stent, biopsy) within 28 days prior to Cycle 1 Day 1, and any toxicity or complications have recovered to Grade 1 or less.
  6. Radiation therapy within 21 days prior to Cycle 1 Day 1. Exception: limited (eg, pain palliation) radiation therapy is allowed if any radiation associated toxicity is resolved to Grade 1 or less, and the radiation is not administered to a target lesion.

Note: Other protocol defined Inclusion/Exclusion criteria may apply.