Easily choose a GAMADV-XTD3 and indicate in the terminal prompt which section that is active
Put gamselect.sh in your path, and edit it to point to where your GAMADV-XTD4 install is residing. Then run it giving your section as input.
gamselect NameOfSection
gam will now switch to the mentioned section and stay there until you run gamselect to another section.
Copy the content of the .basrc to the end of your own, and gam will add your current section to the prompt.
Adjust the path and PS variables as necessary to suit your system.
Now, the way I use it is to create aliases to the sections I use the most.
alias d='source $HOME/bin/gamselect.sh default'
alias ksab='source $HOME/bin/gamselect.sh ksab'
alias lomma='source $HOME/bin/gamselect.sh lomma'
alias nsnet='source $HOME/bin/gamselect.sh nsnet'
Meaning I don't have to type the actual gamselect command.
I just type my shortcut words. d, ksab, lomma or nsnet.
And it'll switch to those sections so I can run my commands.
When I'm done I run d to get back to the DEFAULT section, which doesn't have any JSONs and is safe.