You might’ve heard of Swagbucks before. It’s a rewards site in which the user can do things they already do, such as searching the web, shopping online and watching videos; as well as other interesting things such as surveys and offers; to earn Swagbucks, which can be cashed out, among other rewards, for a free Steam Wallet card. You can then redeem the card to your account and use the credit to get Steam for free.
While, as mentioned before, you will need to put in some effort to get GTA V with this method, but unlike begging your friends and entering giveaways, it has guaranteed results. In fact, you’ll probably get the game faster this way due to the sheer ineffectiveness of the other methods. (By doing pretty much nothing at all, you can still get a $5 Steam Wallet code each month.)