There is no repl, but there are projects in development that make the workflow less of a chore. My own project, SRVi (https://github.com/ajhager/srvi), serves up your GopherJS project, recompiling and embedding when you refresh the page. It will even report compiler errors right in the browser.

Still plenty of room for improvement, but there are people who are excited about making GopherJS fun to work with.