Study to Evaluate the Safety, Pharmacokinetics, and Activity of GDC-1971 in Combination With Either Osimertinib in Participants With Unresectable, Locally Advanced, or Metastatic Non-Small Cell Lung Cancer, or With Cetuximab in Participants With Metastatic Colorectal Cancer

Inclusion Criteria:

* Evaluable or measurable disease per RECIST v1.1
* Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
* Life expectancy of ≥12 weeks
* Adequate hematologic and organ function within 14 days prior to initiation of study Inclusion Criteria for Non-Small Cell Lung Cancer Cohorts
* Histologically confirmed unresectable, locally advanced or metastatic adenocarcinoma of the lung that has progressed on/after prior treatment with third-generation epidermal growth factor receptor (EGFR) inhibitor (e.g., osimertinib)
* Positive for an EGFR exon 19 deletion or exon 21 L858R mutation
* Negative for acquired on-target EGFR alterations Inclusion Criteria for Colorectal Cancer Cohorts
* Histologically confirmed metastatic adenocarcinoma of the colon or rectum that has progressed on/after prior treatment with an EGFR inhibitor (e.g., cetuximab or panitumumab)
* Negative for kirsten rat sarcoma viral oncogene homolog (KRAS) alterations
* Negative for neuroblastoma RAS viral oncogene homolog (NRAS) alterations
* Negative for proto-oncogene B-Raf (BRAF) V600E alterations
* In lieu of a fresh pre-treatment biopsy, a recently obtained biopsy performed after completion of osimertinib therapy will be acceptable

Exclusion Criteria:

* Treatment with chemotherapy, immunotherapy, biologic therapy, or an investigational agent as anti-cancer therapy within 3 weeks or 5 drug elimination half-lives, whichever is shorter, prior to initiation of study treatment
* Treatment with endocrine therapy within 2 weeks prior to initiation of study drug, except for hormonal therapy with gonadotropin-releasing hormone agonists or antagonists for endocrine-sensitive cancers
* Significant traumatic injury or major surgical procedure within 4 weeks prior to Cycle 1, Day 1
* Positive hepatitis C virus (HCV) antibody test at screening
* Positive hepatitis B surface antigen (HBsAg) test at screening
* Known HIV infection
* Clinically significant history of liver disease, including viral or other hepatitis, current alcohol abuse, or cirrhosis
* Uncontrolled hypercalcemia
* Substance abuse, as determined by the investigator, within 12 months prior to screening
* Poor peripheral venous access
* Inability or unwillingness to swallow pills
* Malabsorption syndrome or other condition that would interfere with enteral absorption Chronic diarrhea, short bowel syndrome, or significant upper GI surgery including gastric resection, a history of inflammatory bowel disease (e.g., Crohn's disease or ulcerative colitis), or any active bowel inflammation (including diverticulitis)
* Serious infection within 4 weeks prior to screening
* History of malignancy within 3 years prior to screening
* Known and untreated, or active central nervous system (CNS) metastases (progressing or requiring anticonvulsants or corticosteroids for symptomatic control)
* Leptomeningeal disease or carcinomatous meningitis
* History or presence of an abnormal electrocardiogram (ECG) that is deemed clinically significant by the investigator (e.g., complete left bundle branch block, second- or third-degree atrioventricular heart block) or evidence of prior myocardial infarction
* Left ventricular ejection fraction (LVEF) less than the institutional lower limit of normal (LLN) or \<50%
* History or evidence of ophthalmic disease
* History of or active clinically significant cardiovascular dysfunction
* History of pulmonary firbrosis, organizing pneumonia, or pneumonitis

Other protocol-defined inclusion/exclusion criteria may apply.