The GD45 is the standard-issue sidearm of GDI Engineers. It is woefully outdated and cannot pierce most modern armor, and therefore is only used as a last resort.
The GDI Steel Talons engineers do not use this weapon. Instead, their Combat Engineer use a higher-powered pistol, which is classified by the Corps of Engineers.
The GD45 is never used when a Engineer is in combat, and can only be seen when the Engineer dies.