well, I was playing some CSS when a buddy of mine told me to join his server, so I did, only to realize we were playing SOCCER! I was like, "WTF, a soccer map??" but when I was playing Garry's Mod after that, messing with the gravity gun, I got an idea: a FOOTBALL mod!just edit the gravity gun so that you can only "punch" stuff with left click if you're holding it, and you can only "grab" stuff with right click if it's reallly close to you! I'd think it'd be fairly easy to make, at least, if you can o ANYTHING about modding, unlike myself :P
Newest version of Garry's Mod already has a map featuring football field, ball and players with gravity guns. Game rules are almost same as you described.