The gemara in Bava Kamma 117b brings a machlokes between R' Elazar and the Sages.
If someone steals (gozel) a field, what happens if the field is then flooded? R' Elazar holds that the thief must replace the victim's loss by buying him another field elsewhere. The Sages say the thief merely says "Here is your land".
R' Elazar holds karka is nigzeles while the Sages hold karka einah nigzeles. However, this is only discussing the concept of kinyan gezeilah. Is the property deemed part of the thief's domain while he is in possession, or is it always halachically viewed as being in the victim's domain? For instance, everyone holds that movables (metaltelin) do in fact enter the thief's domain even though the object rightfully still belongs to the victim. This "entering of the thief's domain" is a halachic status of domain regarding accidental loss.
However, the Sages have a drashah from a verse which excludes land from being treated as having entered the thief's domain regarding accidental loss.
Everyone however holds that land can be stolen by using it without the owner's permission. This means that it is a sin to use the land without permission.
The gemara in Sukkah 31a is saying that whatever R' Eliezer (not R' Elazar) holds regarding the question of karka einah nigzeles, a person does not fulfill his obligation to sit in a sukkah by invading a friend's sukkah without permission. If karka is nigzeles, then the sukkah is stolen property. It may be in the thief's domain for responsibility, but it still essentially belongs to the victim. R' Eliezer holds that a sukkah needs to be "yours" ("Lecha") like a lulav. If karka einah nigzeles, then the sukkah never enters the thief's domain. The sukkah might not be halachically viewed as switching domains, but it is at least "borrowed" without permission. This also does not qualify as "your" sukkah.
This seems to be Rashi's explanation on Sukkah 31a.
I hope this helps. :)