Not sure what you mean by 'gas tube'. There is no gas tubes on HK's.
The brass will be absolutely filthy- no doubt.
As for the stuck bolt. If you remove the bolt/carrier assembly and push the bolt back, the rollers will pop out. Easiest way to get the whole schabang back in the gun is to reverse it and cram it into the back of the gun. That will pop the rollers back in and push the bolt forward. Remove, reverse, and reassemble. On one of mine, I can just pull the bolt forward by hand and don't need the 'cram' move.