People used GES maps and models in GMOD before, so this is possible if you know how to add stuff to GMOD.
But having the whole mod in another Game won't be possible.
You can have some gmod like stuff in GES though, e.g. by using prop spawning commands or even scripts.
We had something like that in previouse versions of GES on our private WN server, but it always adds the possibility to crash the server if you spawn to many props.
If you run your own server, you can enable lots of stuff by using sv_cheats 1 in console.
You can also do things like:
bind v noclip
bind F1 prop_physic_create"path/to/physic_prop.mdl"
bind F2 prop_dynamic_create"path/to/dynamic_or_static_prop.mdl"
