There are now several airlines operating the ERJ145 for United. ExpressJet, Transtates, and Commutair. Each is an independent airline with its own carry on bag policy which, while similar, may not be identical.
As always, some agents and flight attendants are more proactive than others at enforcement.
I always gate-check my rollaboard on Express flights but not because it won't (necessarily) fit--it usually would. I gate-check because there isn't much room and if everyone tried to bring a rollaboard aboard the overhead space would fill up very early in the boarding process.