doverin: You’re probably right that it’s not gezailah; see Tosafos to Bava Kama 62b d”h ???? ?????? ??? at the end.
Kofin al midas sedom would only apply after the fact; we do indeed pasken (Rambam hil. Gezailah 3:9) that zeh neh’neh v’zeh ain chaser is patur (from paying), and according to the P’nei Yehoshua on Tosafos on 20a in Bava Kama that would be the reasoning – kofin oso al midas sedom.
See Tosafos on 20b (d”h ?? ???????) as to why kofin oso al midas sedom is a comletely independent din when it’s concerning preventing someone from parking there than when it applies to making them pay after the fact.
The Rambam does not say (correct me if I’m wrong) whether or not Gezel Akum is asur, but the Gemara in Bava Kama on 113b discusses it (saying it’s assur), and the Rambam leaves it out.
In the first halacha in hilchos Genaivah, though, he paskens that genaivas akum is assur mid’oraysah.