Gravis left the Unrestrained Sect until he was outside Liran's Spirit Sense range. Gravis didn't want Liran to find out how he would become a Late Minor Circulation Immortal or the fact that he actually already was one.
Gravis still remembered that this world was different from a beast world. When humans saw someone very outstanding, they believed that they were a genius. Yet, if that person did something that seemed physically impossible, they might decide to kill that person to find out their secrets.
Gravis' unreal Battle-Strength was definitely suspicious, but it could still be explained with all the different Laws he knew. However, if they knew that Gravis could control and absorb lightning, everything would collapse. Controlling someone else's element was impossible and broke the very rules of reality.