This article makes my head hurt. First, the Gefangener GM is a separate unit from the GM Camouf. Second, I'm not sure where that claim came from but there is nothing to support the leg and booster similarity in the mecha dossier from the MS IGLOO data book or from the manga itself. The line in the data profile says "Although it is said that the Zaku system was used from the power pipes fixed on its back to the values of the generator output, these opinions are only speculative."
According to them, the only mobile suit with fram that's slender enough to pass for a GM would be the Zudah (hinting that the Zaku II is slightly bulkier than the Zudah), but there still isn't enough tech specs and history on the GM Camouf.