It looks like IPV6CP was never properly integrated into GAtPPP logic. At least I don't see ipv6cp_new() inside gatchat/gatppp.c anywhere. You might have to extend gatppp.c to manage IPV6CP as well.
I will look into it.
Do you think is a difficult task?
I extended several ofono interfaces fot telit modems.
I will find the time to submit some changes sooner or later.