Has GM ever been on sale before?

Garry doesn't manually put the game on sale but it goes on sale automatically during sales like the Steam Summer sale and the likes.

I always see gmod in sale with Holiday sales such as Winter sales, summer sales etc.

Yes. He was on steam sales. But the discount was not big, as I remember. Somewhere 10-15%

i dont think so it will be happen

Of course! It is always on sale during major events like Christmas, Easter time etc. Just add it to your wishlist and you'll be informed when its on sale.

i bought it on a summer sale

not sure if i can post links but you can see past deals historically here and i think you can sign up to be notified when a game does go on sale: https://isthereanydeal.com/game/garrysmod/history/

It's on sale now. I've bought it for 129 rubles, but now its 124.

yes good sale right now buy while its hot

It's been on sale many times, most notable during the Source/Valve games sale where all valve developed games on source engine go on sale and are bundled.