A Study to Evaluate the Effects of GDC-4198 in Combination With Letrozole, Fulvestrant, or Giredestrant in Participants With Locally Advanced or Metastatic Hormone Receptor-Positive, HER2-Negatve Breast Cancer

The study will evaluate the safety, pharmacokinetics, and preliminary activity of GDC-4198 when given in combination with endocrine therapy (ET) (letrozole, fulvestrant, or giredestrant) as first-line treatment to participants with locally advanced or metastatic Hormone receptor (HR)-positive, Human epidermal growth factor receptor-2 (HER2)-negative breast cancer

Inclusion Criteria: * Histologically or cytologically confirmed adenocarcinoma of the breast that is locally advanced or metastatic and HER2-negative disease. * Disease documented HER2-negative tumor according to American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines or documented ER-positive tumor according to ASCO/CAP guidelines. * Measurable and/or non-measurable disease per Response Evaluation Criteria in Solid Tumors, Version 1.1 (RECIST v1.1) (including bone-only disease). * For females \ 60 years of age. * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1. * Life expectancy \>= 6 months. * Participants with Endocrine therapy (ET)-resistant disease and ET-sensitive disease as defined in the protocol. Exclusion Criteria: * Advanced, symptomatic, visceral spread at the time of study entry that is at risk of life-threating complications and for which treatment with cytotoxic chemotherapy is considered appropriate per local treatment guidelines. * Symptomatic, untreated, or actively progressing CNS metastases. * History of systemic anti-cancer therapy for unresectable locally advanced breast cancer or metastatic breast cancer. * Malabsorption condition or other gastrointestinal (GI) conditions/surgeries that the investigator assesses may significantly interfere with enteral absorption. * History of malignancy within 3 years prior to screening, except for cancer under investigation in this study and malignancies with a negligible risk of metastasis or death. * Known allergy or hypersensitivity to any component of the study treatments.