Gnats in kitty grass

I've been growing Oliver oat grass from seed for years now and he loves it... I used to let it sprout by the back sliding door then bring it up to my room but I've recently found it lasts much longer if I just leave it by the door... well I had noticed in the past that when the grass would get brown (haha I'm a procrastinator) there would be some gnats flying around it... well now it's by the door and still very green and good but there are gnats all over! Any way to prevent them or can I put something near the grass that would attract/kill them? I've tried just now taking a glass custard dish and putting sugar water in it with a drop of liquid soap - I know that works for some bugs... ideas?