I'm going away for some time and have a gift for my girlfriend. How would I use NLP to anchor the gift so that she misses me, when she sees it?

That's very easy mate.

First of all, masturbate and shoot everything on the gift. Secondly, make sure the gift is covered all in sperm. Wrap the gift in the condom that you mainly use/used to use and here you go!

It would be great if someone could give a sincere answer. It is very important to me.

I know next to nothing about NLP, but generally my guidelines for gifts are:

1) Err slightly on the side of too little. More definitely isn't always better.
2) Something she'll use often and have positive thoughts of me/us.
3) Relate to an inside joke or something only she'll get whenever possible.
4) Generally the less she's expecting it, the better.

Nowhere near an expert, but the above has worked fairly well for me. The biggest mistake I've made honestly has been ignoring #1. It seems like the bigger the gesture, the worse it was received because it either signaled neediness/me having to buy her love, or she got something much smaller/less thoughtful and felt bad about it.