Jabber protocol naming is deprecated, XMPP is what it's called now
Gajim still refers to the network protocol as "Jabber". This is the old name of the protocol, it is now named XMPP.
To be technically correct, everything should be revised towards this.
This can cause some confusion to some people, as Jabber is the name of the current most popular XMPP network (jabber.org), so it might appear to some that Gajim only supports that particular network, which is not the case.
The Jabber Software Foundation also has been renamed to the XMPP Foundation when they changed the network protocol name some time ago.
You can find more information about this on the Wikipedia page.