This is a trick question because the answer is simple unless you’re thinking it’s complicated. The short answer is: casual attire. Exactly what you’d wear if you were going to a bridge game or board game night at your local church, school, community center etc. Here’s the tricky party. I only bring this up because some people, especially people who have only heard of D&D or watched it played on TV confuse roll playing games with LARP (Live action roll play) or Cosplay. I’ll make it clear. There are no costumes involved in roll playing games. None. In over forty years of gaming I only dressed up once for a game and that was to Troll a friend of mine at the game, it had nothing to do with the game per se. (short version: Week one my friend says I can’t make a desert character because there are no deserts around in the area. Week two, our first mission is in a desert. Week 3 I wore a desert camo T-shirt to the game.) LARP has some things in common with RPG’s but I’m no expert on that, it’s a different game. Too much walking around for me. No for attire, treat roll playing games like game night. Dress casual, bring snacks and relax.
P.S. When I say game night, I mean bridge or canasta, not baccarat at the Casino Royale. Some card games at casinos can get pretty formal. I’ve never played an RPG in black tie. You could I suppose, but why?