lol I hate that opening too.Surprise your opponent by playing e6 and d5!
How to play against Grand Prix?

I play 1e4 c5 2Nc3 a6. I does well on the databases and I've found it useful myself. The idea is to play e6 then d5, perhaps placing a knight on f5 via e7 if white opts for closing the centre.

Possibly e6
(Edit. Also just checked opening explorer. Black is murdering white according to the stats on the Bc4 line. Quick look suggest as I suspected, d5 themes for black driving back the bishop with tempo...)

Just play e6 and d5. Most players who play these kind of openings are stuck in their autopilot-opening where they repeat typical attacking schemes over and over without taking the current position into account. By playing e6 and d5 you throw them out of their comfort zone because they don't get that autopilot mating attack.
Possibly of interest:
Starting Out: The Sicilian, 2nd Edition
https://web.archive.org/web/20140627122350/http://www.chesscafe.com/text/hansen123.pdf
Anti-Sicilians: Move by Move
http://www.jeremysilman.com/shop/pc/Anti-Sicilian-76p3921.htm
Sveshnikov vs. the Anti-Sicilians
https://www.newinchess.com/media/wysiwyg/product_pdf/9003.pdf
http://www.jeremysilman.com/shop/pc/Sveshnikov-vs-The-Anti-Sicilians-76p3890.htm
Grandmaster Repertoire 6A - Beating the Anti-Sicilians
http://www.qualitychess.co.uk/ebooks/BeatingtheAnti-Sicilians-excerpt.pdf

I play 1e4 c5 2Nc3 a6. I does well on the databases and I've found it useful myself. The idea is to play e6 then d5, perhaps placing a knight on f5 via e7 if white opts for closing the centre.
lol i do the exact same setup as you

I like a setup with Nc6, g6, Bg7, e6, and Nge7. The e6 and g6 pawns help stop an f5 break, your bishop isn't blocked by a knight, and the two knights and DSB harmonize well together in fighting for the central squares.
At some point you prepare b5 as well.

yeah I just play 2... e6 and go from there. Tbh if I was a grand prix player I wouldn't even enter the grand prix after 2... e6, black has a higher winrate than white here... I'd play 3. Nf3 and transpose into a regular french sicilian. I've never found the grand prix that hard to face after 2... e6. But 2... e6 works for me because I sometimes play the french sicilian... if you don't ever play a line in the french sicilian this transposition could pose problems for you. The french sicilian also scores really well though, better than the most popular sicilians... it's worth playing.

Nowadays Grand Prix is becoming very popular and I am having a lot of problems facing it. I am afraid that even low rated players started it. Can anyone suggest me a line/variation against it? Thank you for your help.