I don't have a specific recommendation for a middle name (others will), but suggest the following:
1) With a 4-syllable first name and a 1-syllable last name, your daughter's full name will "flow" best off the tongue if the middle name has 2 syllables (example: Marie, Louise, Erin).
2) Try to avoid a middle name that begins with an A sound so the first and middle names don't blur together when spoken.
3) Gabriela with one "l" should cause no problems or confusion. It's spelled that way in some parts of Europe and certainly occurs in the US.
4) A favor, please: for the love of Gawd, don't use the nickname "Gabby"! Along with "Missy" for Melissa, it's one of the uglier nicknames around (in my opinion). I'm old enough that it reminds me of Gabby Hayes, the grizzled old bearded character actor ("Howdy, Buckaroos!")who played prospectors and cowboys' sidekicks in movies back in the 1930s-50s.