I'm not a "Spain-expert" but found that Gasteiz & Gamarra Mayor & Vitoria are all very close to each other. 
There is a convent of the Poor Clares called Saint Anthony.  
(Convento de San Antonio, General Loma Plaza 7, 01005 Gasteiz, Araba, Espana) 
Phone: +34945233669 Email 
clarisasvitoria@gmail.comThe convent has been largely absorbed by the city. If there was a burial site then it may have been closed and cleared. But you could write/call the convent and see if they can help you, although they are solely a contemplative order. 
You could also approach the municipality of Vitoria-Gasteiz (Ayuntamiento de Vitoria-Gasteiz, Plaza España 1, 01001 Vitoria-Gasteiz, Espana) 
Phone: +34 945 16 11 00 Email 
informacion@vitoria-gasteiz.org