If you want more like this then here's all the Fallout 4 Legendary and Unique Weapons and Armor you can find.
The Fallout 4 Cryolator is an early tease in the game - it's hard to miss as you leave Vault 111, just sitting there, locked away in a glass case, teasing you.
While you can't do anything the first time you find it (a glitch where Dogmeat could simply 'fetch' the gun for you but has been patched out), actually getting the gun is basically just a case waiting. Specifically, you just need to be level 18 with a Perception of 4 to unlock the Master Lockpicking skill that will let you open the case. You should hit that point around Level 20 - 18 for the basic level requirement and another two for the Advanced and Master Lockpicking ranks you'll need.
Another option is to find Cait in the Theater District of Boston in Fallout 4. She's in the Combat Zone arena there and can join you as a companion. She can also be commanded to open any lock, which will also get you the Cryolator by popping the lock on the case.
Once you do get it, here's what it looks like in action:
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