There's some companies that have said you can commentate their game with no problems (Blizzard)
There's some that will sue you for even something as small as putting their system startup sound in a video (Nintendo)
And most of em are somewhere in the middle.
You're gonna have to be specific about the game you want to upload.
Find the maker, the publisher and then google to see if they've made statements reg this or sued ppl.
You might also want to look up "Fair use policy" and, if you're going to do let's play style videos, the concept of "transformative".
If other people are making money, that's their business and they'll have to deal with the consequences should any arise. Same would go for you. If a game developer says, "Hey! You can't do that", then you'd probably be forced to take the content down and maybe pay reparations, but that's up to the developer. On the other hand, they may think it's just free advertisement.
So, for now to be on safe side...i should just upload videos but not monetise them...right ?
Most youtubers that post gameplay videos are partnered by companies like Machinima that allow them to post their monetized content. You won't be able to get a partner when you are just starting out so you just have to grow your channel, it will be fine to keep your videos monetized as you will be making little to $0. The only time a company would ever try to engage you is if you are getting bigger, but by then you would probably have been partnered so it doesn't matter. You're fine either way man.