what do you think of the Grand Prix attack?

It's good if you're prepared to go on the attack immediately, play aggressively, and be willing to lose one or more pieces. Definitely not for the faint of heart.
@1
"I heard it's a good anti scilian weapon"
++ It is. There are 2 ways to play it: caveman style with Bc4 or positionally with Bb5.
"it better to play the alapin or closed scilian?" ++ About the same.
Closed Sicilian >= Alapin >= Grand Prix Attack, as judged by appearance in top events.
"I do play the Vienna, and I was told the two openings are like cousins"
++ Vienna with 3 f4 would be similar. Vienna with Bc4 would be similar to caveman GP.
Vienna with g3 would be more similar to the Closed Sicilian.