Since with mod_rewrite you'll be getting the individual values in variables, it should be easy. On page (in the PHP), just transform them to their database formats and compare them to the results returned by the query for gameID. If you don't have a match, display a generic error or redirect them.
Just remember to sanitize URL data before you use it in a query and you'll be fine.
EDIT: Sanitize ALL data before you use it (in a query or in output), of course, just pay extra attention to the URL here.
I didn't write the PHP myself, and I have very little php knowledge. I designed the database and I can usually figure out what the php is trying to do, but when you say "transform them to their database formats..." I'm not sure what you mean.
my main concern for this is I just recently had to change a couple of dates for the games. There is a chance for duplicate entries for a single game since both dates would work and the page would load either way. I'm trying to figure out a way to return an error if the date or the opponent doesn't match what is listed by the gameID.