Gamzee didn’t actually care about owning any fancy or expensive clothes at all. He just knew that one day one of the scary looking drones went to leave a package at his door and that package had clothes in it. It had six sets of clothes and that was all he had been wearing until he met Karkat.
Karkat was fed up with Gamzee’s lack of other clothes, not because he cared about how he looked, but because Gamzee was outgrowing them (Alternia gave you the first set of clothes and if you survived outgrowing them you were expected to be old enough to order new ones) and Gamzee hadn’t even bothered..
Karkat decided to order new some from his account, but because it was him the clothes came in gray with some red (Nobody suspected anything anyways as red was a famous color for trolls to wear from all hemospectrum). He was able to get a custom seagoat printing on it though. When Karkat showed Gamzee the sweater Gamzee was ecstatic because now he would have something that reminded him of his palebro all the time.
It’s Gamzee’s favorite thing to wear really, to the point where it basically substituted his t-shirts. The pants came later and that was done through Gamzee’s account so that he could at least have his sign somewhere.