Additional Study Information: The purpose of this study is to learn how well RP-6306 (study drug) in combination with gemcitabine works to treat certain types of cancer. RP-6306 is a drug that is designed to stop certain cancer cells from growing by blocking a protein called PKMYT1. It is an investigational drug, which means that it has not been approved by any regulatory authorities, including the FDA. It is taken by mouth. RP-6306 (alone) has been given to patients in an ongoing study to understand how safe and effective it is. This is the first time RP-6306 is being given with gemcitabine to humans. Cancers include bladder cancer, breast cancer, colon and rectal cancer, kidney cancer, liver cancer, lung cancer, melanoma, pancreatic cancer, prostate cancer, and stomach cancer.