I think you should never hide what you do and tell her everything about it.
3 thing can happen:
1, she thinks its gambling and she dont wants to get involved with a gambler --> its useless to try to proof her, she will never understand, she will hate it and you would just always argue about it. she would hate poker even more because she would think its more important for you. if this happens, forget the girl, wohrt nothing to try about it, another will come
2, she dont knows too much about it, but she is open minden and wants to learn it, or at least understand it --> this is a good sign, but it can still happen she will missinterpret it. you should let her know its your "job", she should respect it. She can only respect it, if you give the same attention to her, and she dont has to feel that a dumb online game is more important in your life
3, she knows what it is, maybe she even plays it, so you can have fun buy learning together --> BINGO 
if you go for not telling her at all, i think thats the worst. she will feel for sure dissapointed when she finds out, and if she sees you want to hide it, it will give her the impression that you have things to hide and it must be wrong if you cant tell it straight away
I think no girl worth to argue about poker, beause most of them is just too dumb to unerstand what poker means, but if you find one who is open minded, than you should try to make her understand it even more, and than you have a good chance.
I also dont like to be with a guy, when i can see he is hiding something before me, or i dont know nothing about it. if he would come up with that lets say he is a professional chess player, than for sure i would give him a chance to let me know what this means in real life. But this only of course, if the guy is hot enough 
