Here is the story. It gets a little complicated.
The Father President of the California Missions, who was initially the Blessed Junipero Serra, is credited with "founding" the missions established during their tenure. The Father Presidents did indeed recommend where the next mission(s) where to be established, supported exploration for the best site (and sometimes participated in that effort), chose the initial missionaries etc. While he was Father President t Fr. Serra was closely and totally involved in the founding of the initial mission at San Diego, said the first mass in Monterey and was deeply involved in the ultimate relocation of San Carlos Borromeo de Carmelo from the presidio in Monterey to Carmel Valley. His degree of involvement in the other varied.
Fr. Serra selected the initial site for San Gabriel but the actual founding was done by two padres, Frs. Cambon (the senior missionary) and Somera.
In our publications on the mission we distinguish between the Founder President (in this case Fr. Serra) and the Founding Missionaries (Frs. Cambon and Somera) and list each on the collector cards we sell.
In the case of many of the missions yet another missionary was often the one who really "developed" the mission. In the case of San Gabriel, Father Antonio Crusazdo was in charge of the building of a great portion of the original church, which is very similar in design to the Cathedral of Cordova, Spain. This is where Fr. Cruzado was born and reared.