I believe Bitcoin is ok for a gift, and I won't advise giving altcoin as gift because Bitcoin is less risky to manage in terms of market volatility and long-term holding.

Since we all know how iconic a birthday gift can be, the girlfriend may want to hold it for some time, more long time if the Girlfriend for not have knowledge about Bitcoin and will be needing a lot of teaching and practical guidance to be able to exchange the Bitcoin or keeping the Bitcoin securely store on a cold wallet.

Whichever way it gonna take some time for the gift to be converted to real use.