Why is this mod so popular?

Garrysmod became super popular do to the fact you can literaly script within the game itself with LUA and people took a huge advantage to this and made crazy stuff like Realistic car addons, Stargates etc you name it they made it.

because you can do all kinds of weird shit. Thats why is so good

Variosity of gamemodes and player base can make your time leave away like my daddy

What question ? DarkRP obviously. This is the most popular gamemode.

Physics engine makes it so worthwhile, really is great to play on multiplayer and easy to understand

it is in my opinion the best game in the world i haven't got bored of it after 4 years surprisingly

Because it is simply amazing
Garrys mod is one of the best ever
Recomendos is StarWarsRP

Because no matter what you wanna do it's most likely possible, and if it's not LUA is quite easy to learn, tho there isn't many tutorials out there.

It's just a way to bring aspects of another game into one so it is more avaialbe to people

I don't know, but maybe for its vast emphasis on creativity and unity with others to make the game your own. With Garry's Mod's Sandbox mode being so simple, and having all of the assets given the moment you start the game, it immediately sparks ideas into your mind, and before you know it, you are 100% into the game and having a blast!