A knife. It's easy to conceal and really all one needs. Grabbing my victim from behind I'd use my strenght to force her down, overpower her and secure her hands behind her back. If I'd need to shut her up I'd press the cold steel against her skin, reminding her about who's in charge. The shivers she'd have from knowing that if she doesn't comply, there will be "consequences" would be an added incentive to take her hard and without mercy.
I don't usually gag my partner, since I like to hear her pleads nor do I like to tie her down completely since I want her to struggle (and I'm fairly certain that all you naughty rapees like to struggle too
)