Please cite your source for your contention that the thief does not have to return the object he stole from a goy. How else would he do teshuva?
He must return it because of Chilul Hashem. There is no obligation MiHilchos Gzeilah, though. See Nesivos 348 for further explanation. The mitzva of והשיב את הגזילה אשר גזל doesn’t apply.
(I don’t remember the whole sugya, nor do I have time to look at it now. There is definitely a discussion to locate source for the issur to steal from a goy – whether it is the regular לא תגזול (Rambam), whether it is a דרבנן (I think the Maharshal holds it is אסמכתא, and the Achronim are medayek that Rashi Sanhedrin 57[?] holds like that too), or if it is an עשה (See BB”K 113). It will make a difference whether you have a mekor for a chiyuv Hashavah. See the Nesivos, who compares it to הפקעת הלואתו.)
See the Rema here.
Although, the Rambam seems to pasken that you must return it, although it might be because of Chillul Hashem. Not sure.