Can I spend valentine's day at the gym instead of with my girlfriend
It's strange. Why are you going to the gym on Valentine's day?
Because I like fitness and women don't like it.
Although you might love going to the gym, You should spend Valentine's day with your girlfriend. If you ignore your girlfriend on Valentine's day she'll be very upset.
Nah, she will be upset anyway  if I don't take her out on Valentine's day since you have to spend money on them to get in their pants and I don't have any.
There are many things you could do with your girlfriend that don't cost any money. And she will appreciated you spending the day with her.