A process gate is a "bolt-on" solution. Rather than modifying the existing process to improve outcomes, you treat each team's activities as a black box and try to improve outcomes by sticking something in-between their activities. Which, naturally, means that it can be a crutch for management whose expertise is management-per-se and whose domain knowledge is shallow.

Process gates create Waterfall. Every process gate is a new step in the waterfall. If one wanted to create a Waterfall methodology, process gates are literally the thing you would use to accomplish that. Which isn't always a bad thing!