Given the quotes round "poker", I assume this is not English language?
* Local laws regarding online gambling.
* Local laws regarding running a site on gambling (don't know if anywhere does this if you're not actually enabling gambling on the site)
However, if you're not enabling gambling on the site then extra focus to
* Where do visitors go to indulge. Do these sites offer any of this?
* Where will you get links from if the local niche is not large? Are you sure the top5 are beatable?
* What percentage of those searches are people looking to play online, and how many just looking for basic instructions for a home game etc (May affect monetisation).
Not my niche. I very much doubt I've exhausted the options.