Hello aleahfrancesca,
Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with adding more than one Gmail account on your Mac. You can add multiple Gmail accounts in the Mail app on Mac. Does quitting the Mail app and trying again resolve the issue? This link shows how: Quit apps on Mac - Apple Support
- "To quit an app, choose App Name > Quit App in the menu bar. For example, choose Preview > Quit Preview (or press the keyboard shortcut Command-Q).
Keep in mind that if you click the Close button in the top-left corner of an app’s window, the window closes, but the app remains open (a small dot below the app’s icon in the Dock indicates that the app’s open)."
If not, does restarting your Mac resolve the issue? Shut down or restart your Mac - Apple Support
We hope this helps.
Cheers!