swoley
Criminally, you are looking at at an assault, probably in the 3rd or 2nd degree (in NY at least). Your state may refer to it as a battery. If this is your first offense, it will likely be plea bargained down to something else. You said only 1 person saw it. Who called the cops?
To be on the safe side, I would move your gear. I don't think the police have probable cause to get a warrant to search your home. It definitely appears that there is no exigent circumstance which would permit a warrantless search. Be safe.
In terms of a civil case, unless you broke his jaw, he had to go to the hospital, or you knocked out some chiclets, you are likely OK. If he has to miss work, pay medical expenses, goes to a shrink because of the mental distress you caused, you could be fucked.

However, if you are judgment proof (i.e., broke), no lawyer is going to take it because PI lawyers take cases on a contingency basis (usually 1/3 of the recovery). The statute of limitations is probably a year in your state, but I don't know for sure.
As far as the gym goes, that is totally up to them. What did the cat do?
Hope this helps