Question: This is really bugging me: why is Gabriella suddenly wearing a red dress during the last song when everyone else is still in the same clothes?
Answer: The girls might have decided to dress up for the game. Earlier, they were all shown supporting each other with little gifts and stuff, and the game was going to run longer than either the decathlon or the callbacks, so they could still have time to change in the bathroom.
Answer: Gabriella was actually wearing a pink shirt and a floral skirt when singing with Troy for callbacks but then is magically wearing a red dress at the game shortly after. However, Kelsi and Taylor are both wearing different outfits so perhaps they changed or the game was rescheduled due to the technical issues?
Answer: Right after they finish the chorus, when they start the next verse, you can see Gabriella taking off her Scholastic Decathlon uniform on the right side of the shot. Her red dress is inside. (She might have done that because Troy took off his jacket in the first song that they sang together).