A visa gift card is fine as a tip.  But certainly the first couple times you visit a prostitute you need to pay cash.  

After you have seen her and she knows you aren't going to rip her off you ask her if using a gift card works.

I think it is very individual and mostly has to do with your Lady's needs at the moment (she may be in need of some rent cash) and trust issues.  

XoXo

Lina