I looked back at all scenes with Gen in Tsukasa Empire and I noticed interesting thing. Gen would never stay on Tsukasa’s side because he NEVER felt comfortable there. He looked actually scared and always with his guard up. He never smiled in genuine way. He kept his distance. He wasn’t fine with Tsukasa’s plan. I actually think Gen wore bags with fake blood not since he found Ishigami Village but since he noticed how messed up Tsukasa is. He wore that to protect himself in Tsukasa Empire and it just happened to be more useful against Magma. I really feel bad for Gen. He looked miserable around Tsukasa only looking for chance to escape.
But then he joined Senku’s group in permanent way and his whole attitude changed. He started to smile in genuine way. He felt comfortable and that let him enjoy spending time with people. He started to mess around without fear of getting hurt by some meathead who wouldn’t understand the joke. Gen himself even said that Senku’s side is the safest place on the world. He is really happy now.