Messages (on Mac OS X) used to let you add a gmail account for jabber -- it appears this is gone now? Is there a way to re-enable it?
Jabber and Google Hangouts was removed from Messages in Mojave. You might consider using Adium.
The last macOS to support Jabber in Messsages version 11.0 was High Sierra macOS 10.13.x. Messages version 12 on macOS 10.14.x Mojave removed the ability to add Jabber accounts. Earlier versions of Messages called iMessages and iChat before that allowed one to add ICQ/AIM/Jabber/XMPP accounts.
It is possible to downgrade by backing up your data and re-installing High Sierra but you would likely be better off finding a different Jabber client. Adium, Pidgin, AstraChat, etc.