This one is interesting. While Marguez may be the official FF romanization, both the official katakana マルケズ (Marukezu) and real-world Spanish point at Marquez ~ Márquez. The name clearly tries to represent Spanish, not an invented language.
Danbooru's romanization guidelines state that we should use the official spellings where possible for non-Japanese or outright made-up proper names.
I find this case confusing, mostly because the official katakana spelling conflicts the official roomaji. Yet I guess T5J8F8 has the right idea, as far as all official FF materials use g. But if the original materials waver even slightly, I would certainly go with Marquez.
If the rules for romanizing non-Japanese names are that we used the official romanization unless it's completely ridiculous...I would propose that "Marukezu" somehow being an attempt at "Marguez" is completely ridiculous. I have to wonder if it was a typo that later materials have assumed was intentional.
My guess was ge, and it was stupid of me not to check it right away. However, I got lucky.
The Japanese Wikipedia gives us katakana renderings like:
Guernica → ゲルニカ (GE-RU-NI-KA)
Miguel de Cervantes → ミゲル・デ・セルバンテス (MI-GE-RU DE SE-RU-BA-N-TE-SU).
That said, マルケズ (MA-RU-KE-ZU) is not correct for Marquez either, since the z is pronounced as an unvoiced in Spanish.Cf. Gabriel García Márquez → ガブリエル・ガルシア=マルケス (GA-BU-RI-E-RU GA-RU-SHI-A MA-RU-KE-SU).Personally I find this research quite fascinating, but the makers of FF have just double-botched it. I do not know whether we should try to correct their mistakes or not.
I would still be thankful if somebody who is able to explained what the Japanese-language Wikipedia says about the issue. The Marquez spelling is mentioned there, but in what contexts?