GTA is a very fun and quite well made game with a good story. However, it is also very unrealistic in some obvious ways, a handheld minigun and railgun for example, or crashing one plane into the back of another plane without crashing the second one, it's a little ridiculous. The maps of the worlds don't add up very well together, with some GTA games being set in copies of San Francisco and New York, and the real life counterparts being constantly mentioned in RDR 1 & 2. Red dead is definitely a little more serious and realistic, there are some unrealistic parts, but in the end of the day it is a more story driven game with a focus on very realistic graphics and setting, whereas GTA is quite based around being fun. The main argument for them being in the same universe is the various moments in which they overlap, like the stone hatchet and double action revolver in GTA online or a book written by J. Marston in GTA, but I think those are just cool but not very important easter eggs, knowing rockstar.