So for the #general (bridged to #freegamer on freenode) it would be #general@irc.freegamedev.net and so on. If you want to enable a persistent connection (Not on by default) you can enable it in the room settings. But you should be able to use the /history IRC command in any case to get the older messages if needed.
SASL auth should (untested) work with an ad-hoc command on the IRC server user that opens on connection. For more info on the gateway see: https://doc.biboumi.louiz.org/9.0/user.html
Ramponia also tested it with the Matrix to XMPP bridge and it seems to work (no guarantees it will not break horribly though), use
- {l Code}: {l Select All Code}
/join #xmpp_#general_irc.freegamedev.net:matrix.org
