Gastal was physically unremarkable, but possessed the unique ability to become invisible - completely invisible, to any known way of sensory detection. He was also extremely stealthy for a relatively large Zoanoid. Even the alien senses of the Guyver had great difficulty in perceiving him.
Besides his stealth ability, he had also been fitted or implanted with an organic control unit - an actual body part of the Zoalord Kabral. This was done so that Aptom, upon regenerating using Gastal's body as a base, would come under the control of the Zoalord.