Is there any sexual activity that nullifies the gardasil shot?

Probably not: Gardasil is a 3-shot vaccine that prevents infection from 2 strains of HPV virus that cause cervical cancer, and 2 strains that cause genital warts. It also prevents anal warts (and maybe anal cancer) due to the same strains, in both men and women. The antibodies one produces from the vaccine flow in the bloodstream everywhere. Other body sites are probably protected, but this has not been proven.

Not specificly: Once you have received the series, you are as protected for one activity as any other. It will not protect against exposures prior to the vaccine.