Raegan Gorbachev is a top-down shooter/stealth game developed by Team2Bit and released on 24 Feb 2016.
The sigil was added on release as it was discovered by the players on day one.
The sigil is on the floor of a hidden room that is accessable from Level 22.
Find the rocketlauncher and shoot the right wall at the end of that level to get to that room as seen here. You have to cling to the walls of the room to not advance into the next normal level. Once outside you go through the darkness to the extra hatch on the right and get an achievement.
Trigger an alarm so the door the enemies come through on the top of the level will open. You can walk through it after that and the piece is out there to the north as seen here
Discovery process notes
We found a video from June where the players have "solved the puzzle" before we had time to datamine or finish it ourselves.
The secret level that you access after the level 22 is stored in the "level43" asset. It loads the sprites and their textures from the "sharedassets44.assets" file (which pretty much only hasthe sigil and piece sprites).
The sigil's GameObject is called sigil 2 and the texture is called sonicMJsymbol.
The piece's GameObject is called reagan-gorbachev-map-piece and the texture is called portal3-washwig-map. It's agreed to assume that the portal 3 name is trolling since the sigil was planted in the game after the GD investigation started and GameDetectives already datamined Unity files for other games and seen the filenames. The washwig part refers to another game by the same developer: Washington's Wig. It featured a guest cameo character from Misfits Attic, the creators of Duskers.