Well, you CAN take the zoom off to check it but, it's a hassle, takes more time, and is not necessary. Primes are smaller and can be taken off much quicker and wide primes can't magnify the gate enough to check it. Although you can look through the lens on lenses tighter than 40mm.
When you check the gate your main goal is to see if it is dirty or clean. If it's clean you say 'Good Gate!' and move on, you don't spray air in the gate. If it's dirty then you call out 'Gate's dirty" or something similar to let the AD know that they need to go again. You then clean the gate with an orange stick or canned air. I've never taken a mag off to clean a gate or check it. By taking the mag off you have a greater chance of removing a hair that could be in the gate, so you'd never know if it was there and would think it was clean.