I'm just wondering if anyone still plays it.

I finished playing it months ago.

I load it up from time to time to get a quick hit of nostalgia or try a mod I haven't seen before. Multiplayer use-to be my bread and butter, but singleplayer is good enough for me now.

This game is very very challenging, took me a few tries to finish the first mission. I'm playing it now and then.

Very true and one of the reasons I loved (and still love) the game. Half the battle is just finding the enemy, but the objectives and maps are very well laid out.
If you like the main game I would suggest any of the major mods released for GR. The character models, weapons, and maps (+ missions) rival the original game.
Not having the expansion packs could be a issue, but there should still be stuff to try.

Just bought it yesterday and I am enjoying every minute of it.

What mods work with the GOG version?

The expansions can't be installed on this version, so you will have to find mods that work with the original only.