This is how the Kollel explained the concept of Gollel on Sukah 23a:

3) [line 31] GOLEL L'KEVER ("GOLEL" AND "DOFEK")

(a) There are three basic opinions as to the meaning of Golel and Dofek:

[1] Golel refers to the wooden or stone cover of a coffin, and Dofek refers to its sides. The sides are called Dofek because the cover of the Mes bangs ("Dofek") on them. (RASHI, TOSFOS DH v'Lo Golel, ARUCH Erech Gelal, OR ZARU'A 2:424)

[2] Golel refers to the gravestone that is above the coffin, which is used as a marker. At the top and bottom of the gravestone are two smaller stones that are each called Dofek. (RABEINU TAM, cited in Tosfos Kesuvos 4b DH Ad she'Yisasem ha'Golel)

[3] When people used burial vaults rather than graves, they made use of a large stone or log as a door, which they would roll in front of the entrance. This door was called a Golel because they would roll (Golel) it. They would also use a wedge-like stone or plank, called a Dofek, to keep the door in place. (OR ZARU'A 2:424, quoting Rav Moshe)

(b) The Golel and Dofek are Avi Avos ha'Tum'ah and are Metamei through Maga and Ohel (and according to Rebbi Eliezer in Ohalos 2:4, they are also Metamei through Masa -- see RASHASH to RASHI DH v'Lo Golel).

I hope you find this helpful. Best wishes,

M. Kornfeld

Kollel Iyun Hadaf