I had a kidney transplant from a live donor who got stomach cancer about a year later, now 3 years since surgery. will i also contract cancer since i got the kidney from the donor?

Not likely: No relation between kidney and gastric cancer.

Not likely: While some malignancies may be related to viral infections, and potentially transmitted by tissue, gastric cancer from a renal transplant is unlikely. As a transplant recipient, however, your immunosuppressive medications and exposure to biologic agents do predispose you to other types of neoplasms. See your doctor regularly.