Kake, I dont have a gf right now, but this is how I see it. If your GF is already aware of your foot and tickle fetish, and your relationship hasnt been affected because of it, to me, you have already won half the battle.
I hope Iam not wrong or off base when I tell you this, but, to me, I dont think it will affect your relationship with her. TMF is simply an online gathering place for people to commiserate about our fetish (es). There is nothing wrong, or bad, about visiting this site. So, in my view, I believe that yes, you should tell her. The length of your relationship and time you have spent with her is another issue here. If you have been with her for five years, then hopefully your relationship with her is strong enough where it shouldnt affect it. If I find a gf in the future who doesnt have to do with TMF, I might not tell her on the first date, or in the first month or two I was with her, but once a bond of trust develops, by all means I would. I would hope that if she loved me and cared for me, she would understand.
This is only my view. I would say, you should tell her, honesty is the best policy, and, hopefully, if you have a good relationship with her, she will understand.
Good Luck,
Mitch