CSGO does somewhat have legendary weapons but they are only melee and Overwatch does not have legendary weapons at all.
CSGO Has rare weapons skins like "Dragon Lore", Asiimov, ETC. The only legendary weapons CSGO Has rare the knives like a Karambit, Shadow daggers, ETC.
Overwatch has a golden skin which is applicable to any gun, but you need to buy the skin first with competitive points. You can buy it for multiple weapons, but you have to purchase it for the gun's you want it to be on. Let's say i wanted a golden Roadhog shotgun, I would go into Roadhog's weapon customization menu and buy it for his gun with my 10k comp points. (I think it's 10k)