For example, you might give a birthday gift or a wedding gift, but a gift that wasn't one of those might be a ______ gift.
What about saying it was an "ad hoc" gift?
The adjective ad hoc means:
made or done without planning because of an immediate need
Just a gift:
- only, simply.
After some more time with search engines and thesauri, I think I answered my own question:
Not occasioned, without occasion or cause.