Is it the rambam the only rishon that say that mesachek ba-kubia (one who gambles) is considered gezel (theft) even if he has an occupation (umanut)? And how can he say that, if the psak in the gemara is like rav sheshet and rav yehuda, and they say, that if he have umanot (an occupation) it is not considered gezel?
Answer:
The Rambam is a unique opinion concerning gambling (kubia). As he writes in the laws of edus, one is only disqualified from being a witness if one gambles and one does not have an occupation. Yet, he writes in the laws of theft that there is gezel miderabanan on any gambling. See Sema (34:40) and Tumim (34:16). It does not run against the Gemara, because Rambam concedes that there is no disqualification from being a witness for somebody who has an occupation. This is not in accordance with the way in which the Tur cites the Rambam, which is somewhat difficult.
Much more has been written about the Rambam, but this is not the appropriate place to go into more details.
