Be sure and check the rulebook as ASPC/AMHR has rules on gowns and what you can and cannot wear. Not sure what you are showing or what association so hard to give advice?
Something you can run in SAFELY. Nothing worse than tripping over a long hem.Sensible shoes (not heels that will catch in the surface.)You are looking for smart, not something that will distract the judge from your horse. Not too much 'bling' as that can look tacky. Hats are not a necessity for evening performances, 'party' hair looks better. Hope this helps. See you there