Here is the full list of all 18 Gateways achievements.
Find the multi gun
Find all health boosts
Find all time extending power ups
Find the rotation gateway gun
Find the time travel gateway gun
Find all jump boosts
Find the mirror
Find the resizing gateway gun
Find the torch
Find the gateway gun
Find the time suit
Complete the game
Find all extra echo power ups
Collect all 500 power orbs
Complete the game with 500 power orbs
Destroy all enemies in the game,... including the recharging ones
Download a Steam Workshop level
Record a solution for a help point in your own level