I recently found a blog post that mentioned making your own embossing paste. I'm going from memory but I believe it was one part white glue, one part white acrylic paint and baby powder to get the consistency you want. I believe she started you with one tablespoon of each and 1/4c powder. Don't quote me on the amounts. But you can mix it to your own consistency.
My guess from watching TONS of videos on the Golden Paints channel on YouTube is that you could also just add powder to gesso to make a close match for embossing paste and mix it up as thick as you like. Maybe thrown in a drizzle of white glue to make sure it stays put. Thats what I'm going to do. I believe she had also tried cornstarch but found the baby powder was finer textured and dried smoother.