You should specify whether you want your env variable to be server side or client side in nextjs the way you wrote it's server side if you want your env variable to be accessable in browser you should add
NEXT_PUBLIC_ prefix to your variable name this way it's accessable both in client and server if not it's server only for protecting sensitive info
so in your case your variable should be:

NEXT_PUBLIC_GOOGLE_CLIENT_ID
Nextjs Docs About env variables