Not technically, he IS his uncle. The bad old grandpa and Kiheon's are brothers which makes bad old grandpa Kiheon's grandpa too. Tattooed guy is bad old grandpa's kid which makes him Kiheon's uncle and therefore Gyeol's grandpa.
Thats how he introduced himself to Gyeol when he got caught during their visit to steal some documents lols
He is not Gyeol grandfather, please refer to chapter 10, and 48-50
He introduced himself that way since he wasn't even sure what the correct familial title was. The bald guy with tattoos and ML's father are cousins. ML grandfather is half brothers with the old guy that just got eaten. Old guy had the bald guy later in life, that's why he's close in age with ML. Gyeol true grandfathers are MC's father and ML's father who was murdered. You can check out chapters 10, 48-50 to get a better idea
I wondered why geol called the bald tattoo guy "grandfather" i thought he was ML's cousin but i just realized he's technically his uncle.