@izzion I believe it should be the same way as any other profile, the TEMP and TMP environment variables. Once the MSA has logged in once, you should be able to load the user's hive and change in the Environment subkey if you wish.
Edit: Oh, right, you may not have a profile since the gMSA doesn't log on interactively... you can try this or using runas to open a cmd, but it's starting to look like uncharted waters. If you're okay with changing the directory to use for Temp space for all users, system environment variables are the way to go, probably.
Or if you have a certain process you want to change TEMP for, you can do it in a parent process e.g. cmd /c "set TEMP=C:\Temp&&service_exec"... like most things in Windows, probably a hundred ways to do what you want and the best depends on the specifics of your scenario.