For the requested brands you listed for GBBR I do not believe aftermarket replacements exist. Everywhere else in the world where GBBRs exist without clear lowers they already come with metal lowers. If you need a replacement part for a KJW gun you usually just contact KJW, however the replacement part in question is a restricted import item. I do know that there are many aftermarket receiver for the WA series GBBRs, although I do not know if there is any compatibility between to the companies for lowers. Unfortunately at this time if your looking to have a M4 GBBR with a metal lower you may have to find a retailer to import a full GBBR for you, or attempt to mod an aftermarket WA style lower to fit properly.
As for for Systema, I do not believe a PTW lower will work for an AEG. Perhaps it could be modded for use with a GBBR however when considering the cost its not really worth it as other less expensive options maybe available.
"Its only a little bit on fire"