i know you have no reason to trust a random stranger on the internet, but please believe me when i say that most of the things people are telling you are complete bullshit.

you can buy steam keys through resellers. yes, there is a certain amount of risk involved, but generally speaking, you're going to be fine purchasing from g2a or cdkeyhouse. (i have no experience with simplycdkeys.) just be sure that you're actually purchasing a steam key from either of those two and not a generic pc key if you want it to be a steam-native game. games are released in other countries for much cheaper prices because their threshold for cost vs piracy is much, much lower. if the key doesn't require you to use a VPN to activate, and if it wasn't obtained illegally (9 times out of 10 this is true--you can pay a slight premium for buyer protection on g2a if you're that paranoid/poverty), your key is not going to be revoked down the line.

i've bought tons of games through both g2a and cdkeyhouse. i also buy games at full price through steam. nearly every game now costs $60 on launch, though, and most of them are not worth nearly that much. i got Mortal Kombat X for $22 through a reseller because I knew it wasn't worth $60; knowing what i know now, i wouldn't have paid even $22 for it.

now all that being said... there aren't any gta v steam key resellers charging less than $55. AAA games are one thing, AAAA games (so to speak) are completely different. outside of the grey market, there's no way for you to get gta v on steam without buying it through steam. at a $5 price differential, you're going to be better served buying the game through steam. if you're willing to forego the native steam app aspect, you could get it for roughly half the price.