A few questions for your question.
-How old is your computer? (Found by opening up your computer, going into the top left corner, and clicking on the Icon > "About This Mac", should include "Chip", "Memory", "Serial Number" [Don't share your serial Number], and "macOS")
-Does gmail work on other devices?
-Does gmail work on another browser?
-What version of safari are you on? (Found by opening safari, entering the task bar and going into Safari > About Safari, should return with version xx.x.x[Replace xx.x.x with your version number])
Once you answer these questions, I can assist further.