As it has been ascertained in this thread...
http://forum.12oclockhigh.net/showthread.php?t=27480
the modification with the DB 605AS engine (and replacement with the Erla hood) meant the loss of any pressurized cockpit capabilities for the G-5 which, although on paper retained its designation, in practice become virtually identical to a G-6/AS.
I guess that thus no special use was probably meant for it, at least no different from the one foreseen for G-6/ASs.