There are various views regarding proper nouns ending in s.
The Times Style and Usage Guide (2011):
apostrophes with proper names/nouns ending in s that are singular, [...] where the final 's' is soft, use the 's' apostrophe, eg, Rabelais' writings, Delors' presidency
The Guardian Style Guide (2007):
The possessive in words and names ending in S normally takes an
apostrophe followed by a second S (Jones's, James's), but be guided by
pronunciation and use the plural apostrophe where it helps:
Mephistopheles', Waters', Hedges' rather than Mephistopheles's,
In both guides pronunciation has an impact on the use of s after the apostrophe. These guides, however, only consider names ending in s. I suspect, given the guiding principles stated in the guides, that these rules could be extended to names ending in z when the z is pronounced s.
In any case, I would write Gomez's as this is not difficult to pronounce.