The British squad had already skipped the previous round of the 2019/20 WEC season at Austin in March, but was expected to make a return at Sebring before the event was cancelled altogether due to the coronavirus pandemic.
Ginetta's sole AER-powered G60-LT-P1 will be joined in the LMP1 ranks by a pair of factory entered Toyota TS050 Hybrids, two R-13s entered by Rebellion Racing and ByKolles' sole ENSO CLM P1/01-Gibson.
High Class Racing has also cut back to a single Oreca 07 in the LMP2 class, potentially leaving Corvette exile Jan Magnussen without a Le Mans drive for the first time since his debut in the French classic in 1999.
Fellow LMP2 squads Rick Ware Racing, which was due to run the field's only Riley Mk. 30, and Performance Tech have also decided to skip the race.
Another outfit to drop out of the entry list is GEAR Racing, which was due to enter a Ferrari 488 GTE for Christina Nielsen and two yet-to-be announced drivers in the GTE Pro class.
Following the departures of the aforementioned teams, three outfits that were previously in the reserve list have been granted the entry for the 88th running of the event.
This includes DragonSpeed's second Oreca entry in the LMP2 class, a third Project 1 Porsche 911 in the GTE Am class and an Iron Lynx-entered Ferrari in the same category.
A total of 60 cars are now due to contest the race, down on last year's 62-car grid. There are no reserves, with the D'station Racing Aston Martin not appearing on the entry list despite having been next in line for a place on the grid.
Matt Campbell, who was due to drive one of the two CORE autosport-run Porsche North America entries in the GTE Pro class until the manufacturer pulled them out of the race, has now been given a reprieve in the form a GTE Am drive with Dempsey Proton Racing.
He will share the outfit's #77 Porsche 911 with Christian Ried and Ricardo Pera.
Elsewhere, Eurasia's Ligier LMP2 entry now has no confirmed drivers, despite having named a full line-up previously with Shane van Gisbergen, Danial Gaunt, Nobuya Yamanaka.
Ben Hanley will also race for DragonSpeed in the LMP2 class following confirmation of Ginetta slimming down to one car, joining Henrik Hedman and Renger van der Zande - who replaces the previously-announced Felix Rosenqvist in the #27 car.
Rosenqvist is unavailable as the Laguna Seca IndyCar race falls on the same weekend.