The goal of this clinical study is to learn more about the study drug GS-2426, and how safe and tolerable it is in participants with advanced methylthioadenosine phosphorylase (MTAP)-deleted solid tumors.

The primary objective of this study is to evaluate the safety and tolerability of GS-2426 in participants with MTAP-deleted advanced solid tumors and to determine the maximum tolerated dose (MTD)/maximum administered dose (MAD) and the recommended phase II dose (RP2D).

Key Inclusion Criteria:

• Participants 18 years of age or older (≥ 19 years old for participants in South Korea).
• Histologically or cytologically confirmed advanced malignant solid tumors, who have progressed on, are intolerant to or are ineligible for standard therapy, or have no standard treatment options.
• Participant tumors are methylthioadenosine phosphorylase (MTAP)-deficient.
• Adequate organ function
• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
• All participants must provide a pretreatment tumor tissue sample.

Key Exclusion Criteria:

• Participants with plans to breastfeed during the study period or within 7 days following the last dose of study intervention.
• Have not recovered (ie, returned to Grade 1 or baseline) from clinically significant adverse events (AEs) due to a previously administered agent or a previous intervention as assessed by the investigator.
• Active second malignancy. Individuals with a history of malignancy who have been completely treated with no evidence of active cancer for 5 years prior to enrollment, or individuals with surgically cured tumors with low risk of recurrence may be enrolled.
• Requirement for ongoing therapy with any prohibited medications .
• Prior therapy with a protein arginine methyltransferase 5 (PRMT5) inhibitor or methionine adenosine transferase 2a (MAT2A) inhibitor.
• Have serious infection requiring antibiotics within 14 days prior to the first dose.
• Uncontrolled concurrent diseases

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