20% only for gameplay seems a little harsh and unbalanced to me. Especially when how easy it is to install gets fully half as many points as gameplay! Shouldn't gameplay be the single most important section?
I personally would take 5% each from some/all of audio, theme and UX and re-assign to gameplay so gameplay ended up with 30-35%. Maybe something like:
Gameplay: 30%
Graphics: 20%
Theme: 20%
Audio: 15%
UX: 7.5%
Participation: 7.5%
Or make all the categories add up to 90% and have a final 10% "polish/judge's discretion" so each judge can effectively go through each section and then give a score out of 10 based on gut feeling.