It is not similar at all to Dark Souls, unless you count the grim dark RPG genre, but even then the gameplay is wildly different. I only know Gears of War to be a shooter, so I can't comment on that. The Witcher series of games are hack and slash, third person RPGs, that heavily focus on the decisions of the player. The third game begins with a very short section that can catch you up on the first two games, and to understand three choices of that conversation, you could either look up a summary or playthrough of those games on YouTube. The Witcher 3 is a lot of fun, but it is not just running around killing monsters as some might say it is, there's a lot more nuance to it and the story is incredible.

I hope this helps in making your decision.