There is XMC-side configurations that you need to accomplish to finish this setup.
That information can be found on page 698 of the Extreme Control User Guide, here:
https://documentation.extremenetworks.com/netsight/8.2/9035980-03_XMC_ExtremeControl_User_Guide_8.2.pdf
Essentially, you need to add the GIM IP to your Control engine configuration, and then configure/add a GIM domain (that includes a Local Password Repository) in which the users will be stored.
Any and all GIM configuration data (aside from GIM machine-specific information like IP, DNS, SMTP and SMS configurations) like Guest Users, provisioners, templates, sponsors, etc., etc., are stored on the XMC server in a repository called a "GIM Domain." You cannot actually SEE the GIM domain on XMC (it is embedded in the XMC database), but once configured, any and all configurations done in the GIM gui are stored within XMC and the GIM domain).
Takes a while to wrap your head around it, I know.
So your very next steps are to log into your 8.2.4.42 XMC server, highlight the engine group (it has to be the same engine group that your Access Control engine resides that you added in GIM), and add the GIM IP on the "Guest & IoT Managers" tab of the engine group.
Then create/add a GIM domain (on the Details tab of the same engine group) and point it to a local password repository (if authenticating users, this should be the same LPR as in your AAA config). Any local users, provisioners, and sponsors will appear in this LPR with a "GIM" checkmark next to them for clarification purposes.
Save/enforce, and then you are good to go.
I've contacted documentation again to have this added to the GIM doc, so people aren't assuming that they only have to configure one side for GIM.
Just like Access control, you configure the engine, then add it to XMC. No different for GIM deployments.
Hope it helps!