On a RELENG_6 system, it seems that gpt partition is invisible until a ‘gpt show’ command is issued. So, currently, I have to put ‘gpt show da0′ in rc.early (yes, that’s dirty).
After checking geom_gpt.c, it seems that its start function does nothing on RELENG_6. But, in -CURRENT, it does something. So, my guess is on -CURRENT it is fine. Need to ask some experts.