I think it is, but currently it's really small. I think I read somewhere it currently onle 5% of all online gambling. It's only around 2000 clicks per day for first hit on google.
Bitcoin could not only be used to power your online casino experiences.
Mobile lottery ticket app JackPocket has officially integrated bitcoin payments into its offering, a move that allows users to buy Powerball tickets.
So yeah, now you can buy Powerball tickets for BTC directly, and I consider PowerBall to be now part of the BTC gambling experience.
More info:
http://www.coindesk.com/tickets-1-5-billion-powerball-jackpot-bitcoin/