A pair of Target trackies and a t-shirt... Really, nobody cares what you wear because nobody there is paying any attention to you or even looking at you. They are too busy doing what they are there to do. A gym is not really a place you stare at other people, so try not to worry. All you will do it over think it and put yourself in an uncomfortable headspace. Trust me, you are the last thing anyone else in the gym is thinking about or looking at.