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Joan
You definitely need to size the linen prior to applying gesso. If you are using acrylic gesso it is equally as important as it is with glue/chalk gesso. The acidity in the oil will creep through to the linen, otherwise. Also, there is a phenomenon known as SID (Support Induced Discoloration) which occurs most notably with Acrylic paint, but may also happen with acrylic gesso. It is the migration of water soluble particles out of the support (canvas or linen) into the ground. It can cause a discoloration moving toward the yellow. Sizing the support with say, Golden GAC 100 or even with another couple of diluted coats of neutral pH glue should prevent this as well. All in all, sizing the support is good practice. (By the way, I paint in oils, but I often use acrylic gesso and find the information on Golden's web-site to be most helpful.)
All the best.
Juan
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