Someone already mentioned it, but I love the idea of using just [name_m]Greg[/name_m]. I know several girls with masculine middle names after their dads/grandfathers. For example, I know a little [name_u]Everly[/name_u] [name_u]Scott[/name_u] and I just think its so sweet and balanced.
Without knowing your style or first name considerations, I think something like [name_f]Sylvie[/name_f] [name_m]Greg[/name_m] or [name_f]Elowen[/name_f] [name_m]Greg[/name_m] would be really beautiful!
Aurora (from Gregory, but also sounds similar to oar/oarsman)
Meg (sounds similar to Greg. Also the scientific name for a humpback whale is Megaptera novaeangliae, it also means big winged new angel or big winged New Englander.)
Nova/Angie (the scientific name for a humpback whale is Megaptera novaeangliae)
Mina (means fish in Sanskrit)
Minali (means fish catcher in Sanskrit)
Mai (means water in Arabic)
Maya (means water in Hebrew)
Mira (means ocean in Sanskrit)
Otter
Bay
Baleen (what some whales have instead of teeth)
[name_m]Just[/name_m] wanted to add [name_f]Nimue[/name_f] which means lady of the lake
along these lines, any month name would work as an honour name because of the Gregorian Calendar!
Gregora
[name_f]Gregoria[/name_f]
Grega
I think even using [name_m]Greg[/name_m] or [name_m]Gregory[/name_m] as a middle name with a feminine first name would also work.