I think you’re stuck with the rockstar launcher either way, at least that’s how it is on steam.
As for how it will run on your rig I think you’ll have no issues. I ran it on my old i7 3730k with a 1080 at 3440 x 1440, with a mixture of ultra, high and medium settings at a good 55 plus fps. I could have dropped a setting or 2 to bump the frame rate but was happy enough with it as it was.
Have a look on reddit for some decent guides for the settings, those worked great for me.