Dov Gutterman forwarded me a link to the Official Bulletin of the Diputación Provincial (provincial council) of Guipúzcoa approving the flag and coat-of-arms of the Ayuntamiento de Gaviria / Gabiriako Udala. I made a GIF using the black and white image there. Some comments:
The law says that the field of the flag is ochre – I have used light brown for it.
The blazon of the coat-of-arms does not actually state the colour of the star, but it appears to be Or (i.e. gold) since it is blazoned at the same time as the crown.
The law does not state the ratio of the flag either. The illustration supplied appears to be circa 11:17 so I have used this, even if most probably it is really 2:3 as most Spanish local flags.
According to the law, the reverse is a plain field.