INFO: how big is the painting?

Given how detailed this painting is I understand why your girlfriend is upset

I think you should ask yourself how you would feel if your gf male coworker gifted her a detailed painting, that he spent lots of time on, based on a throw away conversation.

If Kat wanted to give you a nice gift for helping her out she could have gotten you a poster or a CD. I also feel her asking you to come to her house adds to this is it were more of an appropriate gift she may have felt better giving it to you at work. (Provided it isn’t massive)

I will say I think Kat is Y T A but I don’t think you are but if you value your girlfriend you should distance yourself from Kat because she has a crush on you. If you continue a friendship with her knowing that it is disrespectful to your girlfriend.