The Gate Glitch, sometimes referred to as "Blue Gate Glitch" if the gate in question involves a blue switch, or "Green Gate Glitch" ("GGG" for short) if the gate in question involves a green switch, is a technique to open gates that can normally only be opened from the left (and without using Wave Beam for blue gates).
This glitch is mandatory for categories such as Any% GT Code and GT Classic for accessing the Golden Torizo code. It is also essential in Reverse Boss Order for accessing Screw Attack, the recharge station nearby, and the Ripper 2s that act as a Super Missile refill.
To perform the gate glitch, Samus must first stand a couple of pixels to the right of the gate while facing left, and have Missiles or Super Missiles selected (whichever is applicable). Jump while holding angle up, then move left and press shot right before her head hits the ceiling (while still holding jump and angle up).
The gate glitch cannot be performed in this manner on gates where the switch is on the right side of the gate. Depending on the gate in question, abusing the camera may be required in order to activate the switch (Pink Brinstar Hopper Room), but many of these gates either have extremely complex solutions to unlocking them from the opposite side (Red Brinstar Elevator), or are impossible to unlock from the opposite side (Crocomire Escape).
It may take a couple of tries before one actually hits the button since the timing is rather precise, so make sure you have enough ammo before attempting the glitch.