This command will give you an engineer of the specified level. You can only have one engineer at a time because XCOM 2 treats engineers as an actual unit instead of counters. So if you already have an engineer, that engineer will be replaced. The level determines the engineer's effectiveness. Higher levels provide higher speed boost in construction.
Level is a number specifying the engineer's level. This number ranges from 1 to 100.
The command GiveEngineer 4 wll give you a level 4 engineer.