Are they looking for nicknames for [name_f]Gabriella[/name_f]? If yes maybe [name_u]Aubrey[/name_u], [name_m]Gabe[/name_m], [name_f]Abriel[/name_f] or [name_f]Bri[/name_f].
Many of the iel names sound gender neutral to me
[name_m]Adriel[/name_m]
[name_m]Azriel[/name_m]
[name_m]Azrael[/name_m]
[name_m]Uriel[/name_m]
[name_u]Ariel[/name_u]
Sabriel
[name_m]Abdiel[/name_m]
forms of [name_u]Gabriel[/name_u]:
[name_u]Gabriele[/name_u]
[name_m]Gavrel[/name_m]
[name_m]Gavril[/name_m]
Jabril
Similar sounding
[name_u]Abbott[/name_u]
Sabriel
[name_f]Gail[/name_f]/Gale
[name_m]Abdiel[/name_m]
In the book Before We Were Yours, one of the boys is Gabion nn [name_u]Gabby[/name_u]. I think it’s unknown enough that it’s not associated with one gender.
I know lots of Gabriels and Gabrielles/Gabriellas from where I’m from just go by [name_m]Gab[/name_m], that strikes me as extremely gender-neutral and intuitive enough as a nickname that no one will question it! Maybe that would work for your friend?
there’s another og hebrew form of gabriella spelled with a v so like gavrielle, gavriella, gavriel, gavra, etc, maybe they’ll like that better? it’s not much of a change but it’s a totally different sound! gavriel is a lil less associated with masculinity than gabriel so maybe that’ll feel more them.
there’s also always gabbe which i guess feels less strictly nickname than gab.
some other kinda similar names that are hebrew in origin too so they may give enough of similar vibe to feel like a good fit! gali/galy geva gebah/gebba gibea/gibia/gibeon gilboa/gylboah
best of luck to you and them! my brother went through a bunch of names before he found his. hopefully they don’t feel stress and instead get to have fun trying out some cool name options!
I don’t know why I keep doing that, I know your name is [name_f]Brynn[/name_f]! 
