The insurance companies will need to use the claims history for F-HD when establishing G-HD rates so I expect rates to be similar.
For attained-age plans, claims history will show a 67 yr old F-HD enrollee typically submits 'X' number of claims with $Y payments. At this time, there is no reason to expect the typical 67 yr old G-HD enrollee to be sicker or healthier. Same for other ages and issue-age plans.
In a couple of states (NY and CT) the plans must be community-rated so I expect G-HD rates to be slightly lower since their "community" of enrollees will initially trend younger and healthier.