i know steam have workshop where you can find diffrent mod for this game. when you have it on gog where can you find those mod then???

On Nexus:

http://grimrock.nexusmods.com/

how do you install them?

Unzip the dat file in

C:\Users\yourusername\Documents\Almost Human\Legend of Grimrock\Dungeons

or, of course, whatever the equivalent of that directory is in your OS.

thx :)

There's also a mod section in the forums at the official site ( www.grimrock.net ). While most of those mods will be hosted on Steam and/or the Nexus, there may be a few hosted elsewhere, and the discussion threads will give you an idea of which ones are new/good/upcoming; some mod makers also request feedback there.

is there any mod that is good to play on the orginal legend of grimrock?

Definitely!! Some don't even feel like the same game anymore. Ones I can highly recommend are:
The LOG Master Quest by Komag
Awakening of Taarnab (and not just because I'm in it :) by Mahric
The One Room Round Robin Dungeon

These and a list of all completed known mods can be found at http://www.grimrock.net/forum/viewtopic.php?f=14&t=4361

Keep an eye on this forum too; my own mod is nearing completion. It's called "The Hub" and has new monsters, sounds, wall sets, objects, and skill-raising books, along with a quirky sense of humor, 20(ish) floors, around 80 secrets, and tons of pop cultural references to find.

As I'm the author :), I can only recommend this: http://www.nexusmods.com/grimrock/mods/277/?