GTA V: Yes, this system will be able to run it (or any game really) pretty close to maxed out up to 1080p resolution, probably even 1440p without too much trouble. For 4K ultrawide, or multi-monitor setups the 3GB 1060 will bottleneck to playable at closer to mid level settings but again if you're only looking to run 1080p this is a great machine.
RAM: The RAM can be any brand, but has to be the spec that fits the motherboard, which in this system is the Odense2-K motherboard from HP. This board has 2 slots for DDR4 RAM, one of which is taken up by the included 8GB. It can be upgraded to a max of 16GB DDR4-2400 RAM (the 2133 spec on this page is incorrect), and I would advise if you were to upgrade to buy a 2x8GB kit so that the speed and timings match; but it won't be immediately necessary as I haven't yet had too much trouble with just 8GB.