Hmmmm.....
Well ive played gmod for well...since it first came out back when it was...8 maybe 9.
Its a fabulous game, seriously a great deal of stuff to do
First off: BUILDING
It all depends on what your building, how complex it is, and your knowledge of how mechanics and physics work.
Simple things like vehicles and forts dont take to much time or expertise.
But say...a working walker, robot thing, that'll take a bit of time and know-how.
Now, theres more to Gmod than just building, I'd say 40% is building, 60% is crazy, wacky, addiciting, fun mods.
They have an amazing slew of zombie mods.
Some RTS kind of mods...i mean theres tons to name.
One VERY important thing to look out for...RP or RolePlay.
This is....dangerous territory. Its a....double edged sword if you will.
Tons of fun can be had, but theres a steep learning curve, and alot of patience and creativity required to be good at it.
Another huge factor, if you get into it, is the addicition. I got sucked into that hole, and played HL2 RP for close to 2 years. So you've got to be careful.
I wouldnt advise going into HL2 RP, its a dieing age, all the vet's and great RP's have moved on. Its alot of minge's now, thats why I quit it at the end of last year.