What is that gray putty behind exterior fixtures?

The gray putty tape was originally called GaskaTape, it was developed and marketed by an Elkhart neighbor of an ours in the early 60's. Try ordering it from Johnson's RV Surplus in White Pigeon, Michigan. On looking it up I was shocked, the last I purchased from one of the parts liquidators about 15 years ago was $2.25 a roll. I actually still have a roll or two left in the shop in a zip lock bag, still good the last I checked. Kinda like one of those Christmas Fruitcakes, lasts forever, if kept in an air tight package or container.