Here is the full list of all 31 Stranger Things 3: The Game achievements.
Unlock Hopper and Joyce
Complete Ivan the Smuggler
Complete Chores for Granny Perkins
Complete Wall Flower
Complete Rats in the Walls
Craft 5 trinkets
Successfully hack the hardest level chest/door
Complete Revenge of the Geeks
Unlock Eleven
Unlock Max
Unlock Jonathan
Unlock Nancy
Complete Store Raid
Complete On the Wire
Complete Break In, Make Out
Complete Radar Love
Crafted your first item
Unlock Dustin
Complete Sneak Peek
Complete Hospital Brawl
Complete Cabin Ambush
Complete Mall Showdown
Complete 30 Minutes or Less
Complete Invasion of Privacy
Unlock Erica
Unlock Steve
Found 25 hidden gnomes. Half-way there!
Unlock Will
Complete the game
Beat the game on New Game+
Found all 50 hidden gnomes.