I had no issues keeping my whole team alive as long as the tanks knew to guard healers and taunt the strongest damage dealing enemy that wasn't targeting the tank (even in recruit gear).
Effectively, with a decent team who plays the roles as they're intended gear isn't really the primary factor. However, if both teams are just running around trying to do their own thing and one team has a bunch of War Heroes, yeah, the gear will be the factor.
Playing effectively as a team is a much larger factor though. I've been in a lot of rounds with Battlemaster and recruit gear against a team with a decked out player or two and while you could tell they died a bit slower and dealt a bit more damage our team still won the matches by a large margin.
My point? Gear matters. Skill matters more. If you think it's impossible to win against a better geared team either you've been unfortunate enough that their team is as or more organized (understandable if it was premade) or you're dragging your teams down.