SUSIE'S REPLY: Hi Vera, I'm not a chemist but I don't think gum arabic would make a suitable retardant for acrylics. It's used as a binder in watercolors because it dissolves with water. I'm sure if it worked well with acrylics it would be promoted and sold by the acrylic manufacturers.
Keep painting and learning! Practicing and painting every chance you get helps you learn from experience.
Thanks for writing!