Hey Mike,
Note this was a trackday, not a race, so there were mostly people drving in stock/street cars, and the only requirement in that case is helmet.
I don't want you to get the wrong impression but, all races that take part here have to comply with FIA safety regs, so no, you won't see guys racing their GT3 cup, or other racers for that matter, in short's and t-shirts
On the other note, yes I'm thinking of getting a proper apparel, but that was not on the priority list this year.