Yes, this is an actual guide for an actual achievement.
How to Obtain Die Achievement
Step 1: Find Greg
Step 2: Equip Gun
Press G to equip gun.
Step 3: Shoot Greg
Left click to shoot.
Step 4: Oh God What Is Happening
Step 5: Congratulations! You Are Now Deceased!
I found Greg by going around in circles after reading you can find him in parts you already visited, looks like he moves around in the areas you visited
Where is greg?
Im not entirely sure, but I think you can usually find greg by searching in parts of the gallery you have already visited.
You can escape ded by fleeing to the nearest exit, TPing helps. Glitches the game BGM though.