For gambling casino , I will recommend to start advertisement here on bitcointalk. The best way is to start the long term signature campaign.
It might be more appropriate to promote precisely that many gambling activities from high traffic sites but only few active gamblers, I think this forum is very appropriate to promote gambling as many active gamblers and gambling campaigns are very popular in this forum.
Actually having a long term signature campaign here at the forum would be a great way to promote your platform however you should also consider other alternatives. As you might not have receive the amount of traffic you're expecting for your gambling platform.
Bonuses, Campaigns, Events and having a great functional platform would be the best way to promote your online casino.