How can I have a positive HPV pap test is I previously had the gardasil vaccine?

Non-vaccine HPV. Gardasil covers only hpv types 6, 11, 16, and 18. There are a number of other strains and you could have one of those. For more information on Gardasil see this site. http://www.cdc.gov/vaccinesafety/vaccines/hpv/index.html.
Many strains. There are no less than 7 strains of HPV that can turn your pap positive & only the 2 that caused it most often went into the initial Gardasil vaccine.( there is now one with the 7) Gardasil can also not treat or eliminate any strain you acquired before you took the series. Continued Pap smear surveillance will be a valuable practice whether you get the vaccine or not.