While most MP5 parts are not interchangeable, most fore-ends should fit.
The HK-style sights aren't great for precise or target shooting; an HK claw-mount will be the way to
go to mount optics on the GSG-5.
The GSG-5 has controls identical to the MP5, even the "HK smack" works, although is exacerbates
cocking tube peening.