Eastbourne guaranteed their survival in London Three South East and sent East Grinstead down but it was a close run thing.
Grinstead led 14-9 at half-time and were boosted by the return of goalkicker Steve O'Reilly but eventually lost 19-17.
Eastbourne's Graham Shiret admitted: "They don't deserve to go down. We have seen much worse teams than them.
"We had to play well to win but I thought we just about deserved it overall."
Stuart Dunk and Andy Hebron scored tries with Ollie Cummings kicking three penalties for the winners, who will be relieved to steady matters after a topsy-turvy season.
Dave Tyson added a try to O'Reilly's four penalties for Gs, who have lost eight games by less than three points this season.
Meanwhile, Lewes's quietly impressive season is drawing to an upbeat finale after they defeated visiting Charlton Park 30-15.
Skipper Dave Hemsley set things rolling with an opportunist score after Matt Sole's kick through had been fumbled.
Flanker Ben Sole was next to dive over, following a driving maul, with Dave Tyrrell converting both tries and kicking a penalty.
Tim Hemsley, on for his injured brother Dave in the unaccustomed position of hooker, scored a third Lewes try but Park got back to 22-15 before Huw James's superb break sent in man-of-the-match Ben Sole for his second score.
Tyrrell kicked another penalty to round things off.
Brighton went down 39-14 at home to leaders Sidcup though that did not really do justice to how they performed.
Mark Gibb scored a try and kicked two conversions with Julian Hutchens also going over.
There is still a lengthy injury list at Waterhall and skipper Mark Jenkins said: "The result does not show how well the players did."
Tudor Ellis earned praise after switching from hooker to tight head.
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