Hi,
In Older versions of iChat and Messages the Google Accounts used the server talk.google.com
This is different to other Jabber servers which use the bit after the @ as the server name (You had to manually change this in iChat 3 - at iChat 4 they added the specific Google Account to get around this.)
The reason fro this was that Google Mail IDs (used for Google Talk) came in two forms (@gmail.com and @googlemail.com).
In Yosemite and the El Capitan beta the App changes this to look like this.
The Server Name has changed to gmail.com (which happens to be the ending of my Google Account.
AS you can see it lists "Automatically Find Server and Port" and uses port 5222 (non SSL as well).
If I sign out and manually change it it reverts to this setting.
I believe that this may be to do with System Preferences > Internet Accounts as the Google account there is listed as active for Messages and Mail (Of course it is @gmaill.com for the mail server).
I had a similar issue with my iCloud account and the names linked to since joining iCloud.
Internet Account made the original Apple ID the iCloud ID which meant the Apple ID I was using in iMessages got changed to one ending @me.com
I have to make changes to revert this to the set up I did have which used one ID and the iMessages ID and another fro the iCloud sync for Contacts and Mail.
If I manually change it in System Preferences (untick Messages and then change the server in the Messages App it looks like it is going to Log in but take a long time). It also overrides System Preferences and Activates Messages again
I may have to Quit Messages and possible delete and re-add the account without using System Preferences > Internet accounts.
I did a Search on Google to see if there was easy access to set up for other Apps and got this:-
Google Talk with Messages Mac
As I stated at the beginning part of the original issue was the naming of the Google server compared to the ID being used.
I would look at that first.
I presume that the ID works in a web Mail login that has been set up to display Chat as well ?
I have also been searching for a web page at Google that tells you the set up for other apps. (There used to be one for iChat that listed the Server name change and I was hoping to find any new server name).
9:35 p.m. Thursday; August 27, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad