Similar-ish. They're both open world games set in California with 3rd person shooting mechanics but I too will have to agree with others that Watch_Dogs 2 is more enjoyable than Grand Theft Auto V.
It has to be said, I hate almost all the characters in both games. The kids in WD2 act like how a committee of 40-year-old men think kids these days talk (which is to say though "hip" dialogue and memes) and so they come across as almost a self-parody of what hackers are like but the game play is hella fun. Even in the first WD, despite Aiden being a miserable, dreary bastard, the core game was fun as hell. They both blend stealth, hacking and shooting fairly well and the gadgets you are given to screw around with (a small quad copter drone and an RC car thing among others IIRC) are incorporated into the game play and exploration well. The driving is a bit...arcade-y though, especially if you've gotten used to the "realistic" driving mechanics of the HD GTA games. Every time I switched between the two games I had to almost unlearn how to use cars.
The setting is also more...focused? In GTAV, you can go driving all over the massive county map but I'm at the point where huge open world games don't impress me much any more since they've become the standard. WD2 is set in San Francisco and mostly takes place within various parts of the city itself. To me, making a rollicking map that goes on forever is just another way of creating a false sense of DIY game play. If I wanted to wander all over a massive map, I could play Just Cause 3 or something. Anyway, the city is pretty unique in WD2 I feel like, and apparently takes cues from actual landmarks in the real San Francisco. The scale of GTAV is impressive but a lot of it is just empty space, so what's the point?
I haven't finished it yet because I bounce around between games a lot but what I have played of it has been great, if you can get over the incredibly cringy characters.