back in the day i also used to drive in after 5 or 6 when no one was at the gate. it was an honour system. put your money in an envelope. lots of people did not. plus, they also charge a fee for fishing on top of entrance. not sure if it has changed in the few years since i've been. in any case, it is almost always not worth the money considering how the fishing is.
one time i did drive in there at 6 and one of the staff did drive down to the lake and ask to see our fishing permit for the lake that day. we had to leave. 9 times out of 10 they won't care to bother you. this is on weekdays.
if you and some buds split the cost of a rental, then it could definitely be fun since it's a nice lake and potential for catching some nice bass during the open season, but most of the time the bass will be on the smaller side.
there are better and free options in brampton nearby if you're afraid of having that awkward conversation if staff ask to see proof that you paid.
and ya, the rainbows are usually small and hard to catch.