A bit late but I just registered here.
LPG in a G55 is no problem, you can do it without loss of power with the top systems available. Prins lpg, a dutch company, is specializing in LPG and I know of a E5 AMG that drives on LPG in Belgium (youtube: LPG amg), which is owned by the importer of Prins systems. Here also Posrsche Cayennes, Dodge Vipers etc run on LPG withpt problems.
The only problem might be that you need an experienced mechanic to put it in your car. Maybe the guys at Prins can help for your region, might be a big problem though. You might just have to send your car to Holland or Belgium
company's website: Welkom bij Prins Autogassystemen B.V.
PS: I have no interest in this company
I only am in the process of getting a G55 myself and then convert it to LPG.