What should I do about not being sexually attracted to my girlfriend of five years?

Probably what you should do is decide whether you can continue having a romantic and sexual relationship with someone to whom you don't feel like having sex.

Five years is long enough to know that the way you feel toward your partner is more than likely going to remain this way.

Talk with your gf about your feelings because she may not wish to be in a relationship in which her partner is not sexually attracted to her.

Sexual attraction is often discussed on a continuum. If you are saying that you are not attracted to your girlfriend, you could try changing what you do during sexual intimacy. You could also try slowly using nonsexual touch and discussing what you each like and would prefer to do more of compared to what is not as desirable. After doing this for nonsexual touching, it can also be used with sexual touching. Looking at more of what you like or would like to feel may be of help.