I have Sins of a Solar Empire Rebellion in my Steam account, but GOG.Connect says that it is not found in my Steam library. At first I wondered if it was because of my missing DLC(in my case, I do not own only OST in my Steam library), but some says that it might be glitch with GOG.Connect.

I have a will to purchase missing DLC if it works with GOG.Connect, but time is running out, so I want to know what the truth is as soon as possible.

Does anybody have answers or simliar cases?

Connect it's currently having problems identifying the Ultimate Edition as owned. It may have something to do with the identifiers used for bundles in Steam and how Connect reads them. The same happens with Moto Racer Collection, Connect doesn't seem to realise it's the same as Moto Racer 1, 2 and 3. Others have also reported problems with Metro 2033 Redux as well.

For the time being, there's little more to do than waiting for GOG to solve it. It seems that GOG is aware, at least, of the problem with Sins of a Solar Empire; not sure about Moto Racer or Metro. Some have found success by accessing Connect from another device: like another computer or a phone. It may be worth a try in the meantime. Perhaps even another browser can help if cookies are also involved. There's really not much to be done for now.

Do take into account that the fact that you are missing the OST can also be a factor. Therefore, if you don't own the OST when Connect starts working correctly you may still not be able to redeem Sins of a Solar Empire on GOG.

I have the same problem although i got the ultimate edition on steam, which includes:
Sins of a Solar Empire®: Rebellion
Sins of a Solar Empire®: Rebellion - Stellar Phenomena®
Sins of a Solar Empire®: Rebellion - Forbidden Worlds™ DLC
Sins of a Solar Empire®: Rebellion - Original Soundtrack

I also tried to log in from other browsers / devices but no luck.

hope someone help us.

Same problem here and I got the whole Sins of a Solar Empire: Rebellion on Steam.

The only DLC I don't have is "Outlaw Sectors". – Still Connect does not recognize it as eligible.

I contacted GoG regarding the Sins Rebellions Ultimate connect issue but so far no response. I hope this will be resolved soon.

Likewise.

Answer i found (and this goes for Redux versions) that GoG connect and their catalogue will not recognise as having the game, and therefore will not recognise it.

Their Catalogue also has issue's as in you may buy a pre order game, but their catalogue would still show as not having it and say you need to purchase it, even though, in actuallity you do and did purchase the game from GoG catalogue.

As with Sins of solar Empire - They only had connect register the Ultimate edition, so if you purchased the game and then purchased teh DLC's seperatly when they came out then connect would NOT recognise as you having sins of a solar empire. This goes for any game that is done like that as well.

They do have issue's that need fixing, but maybe they dont have the ability (or their software does not) to properly do this. I would have therefore thought Staff would manually sort things out once informed.

They have been. If you haven't got the game (and should have done) then you can contact support and they should sort it out.
Check this post by fables here: https://www.gog.com/forum/sins_of_a_solar_empire_rebellion/gog_connect_not_detecting_sins_in_steam_library/post58/?staff=yes