Best places to advertise a BTC gambling would be in a BTC gambling review sites. People are looking for reviews before trying on the actual sites

Having the review sites to review your sites will be consider as an advertising since if the review of your site is good then you can expect more people to try your site. Some BTC gambling reviews site issue up an advertisement slot as well and this could be your opportunity if you want to an extra advertising at their homepage

I would suggest two sites to you

#1 https://thebitcoinstrip.com/

#2 http://cryptobettingindex.com/

Both sites are great for advertisement and bitcoinstrip has been doing honest review for all the sites here.

P.S : some sites may be biased on their reviews depending on what you "pay" them