Hello
The site Geopatronyme.com shows that your family name Gavelle is most common in the département of Pas-de-Calais, in the cities of Calais and Boulogne-sur-Mer, so you are definitely on the right track! Please see this link:
http://www.geopatronyme.com/cgi-bin/carte/nomcarte.cgi?nom=G...The Archives départementales du Pas-de-Calais has 10-year indexes from 1793 to 1902, censuses to 1906, and military matriculations from 1867-1930 online for free at this site:
http://www.archivespasdecalais.fr/Note: The indexes, whcih are divided by town, and by marriages, births, and death, by 10-year period are easy to search. They have not put the digitized images of the actual certificates online yet, but it is anticipated that they will be there within nine months.
The best place to collaborate about French genealogy in GeneaNet. It is the largest site of user contributed trees in France, with much, much more. It is a great way to contact distant relatives. All basic functions are free. The site is in French and English (click on the flag):
http://www.geneanet.org/Bonne recherche !