I wish I had a meaningful GF who would wear my Oxford shirts to run errands the day after she slept over at my studio apartment. These large button-up shirts are required to be XXL in order to function as a 'dress.' It seems that she is on her way into a Tailor/alterations shop in order to fix her bf's shirt, though. Hopefully she doesn't make any changes without his consent. Hate it when BFs/GFs do that!

The last girl who wore her BF's shirt wore leggings and didn't utilize a belt. I wonder which 1 is better for an on-the-go lifestyle.

[Photo from Style Scout]

Would it be offensive to buy ur alternative GF these as a Birthday Gift?

Maybe if u include a pair of leggings, she'll appreciate u for 'truly caring abt the way she looks.' Sit down with your partner and discuss how 'conveying your alt-ness with fashion choices' is an important element of life that both of you value. Don't just get her a Chili's giftcard--she already expects you to take her to Chili's on her BD. That's just part of 'being serious' with some1 special.
(Choose.Your.Own.Appetizer)