Advance prep and storage: You may assemble the stromboli up to a day in advance, cover well, refrigerate, and bake right before serving. The unbaked strombolis freeze well too. Thaw before baking. If freezing, wait to brush the top with the egg until just before baking. Leftover baked stromboli will keep in the refrigerator for 4-5 days and may be frozen.
Gluten-free? I recently tested Wholly Gluten-Free Pizza Dough at our local Wegmans (and likely other stores), and it worked very well. (My non-GF brother texted me and said he liked it better than the wheat version!) The loaf is 14 ounces rather than the standard 16 ounces, so take special care not to overfill. I did use the same basic proportions, going ever-so-slightly scant on the fillings. When baking for gluten-free diners, be sure to also use a gluten-free flour when rolling the dough.
Egg allergy? Be sure to seal the edges well by gently pinching, and then brush the top with melted butter or olive oil instead of the egg.