tell her
My 2 cents as a woman: tell her.
I liked the idea about telling her gradually, though. Warm her up to the idea. My last girlfriend was against the whole herbal/supplement thing, but I brought her around slowly, gradually. Gear is scarier, but still...
Try Dhea and trib or whatever, openly, and, when those don't work, tell her you want the gains, but don't want to stack up those products because they, the legal ones, have worse sides than the ones that are illegal.
It takes more time and tact, but it'll be worth it. Most women I know (myself included) don't like to have something huge sprung on us, but can be a lot more reasonable when it comes to gradually moving toward something.
That's if you value the relationship. If you aren't so serious about her, then either tell her straight out you're doing it and let the fur fly, or just break up with her.
Lying is the worst, though, especially about such a big part of your life...
Wyst