I'm not sure about GSM4 but GSM3 is already feature ready to be sold on Steam.

The question is: why hasn't Matt done this yet?

After checking with Matt, I found this is a particular problem with my OS.

I fixed it by restoring to an earlier time.

Something has broken the program with the 23 Feb 2019 update, now you cannot restore save games via .gbsa file, at least for Ultimate General: Civil War.

The archive still works and you can manually copy the saves by extracting it though. Very strange, please InsaneMatt, recheck it.

Edit: I check with another game: Armies of Exigo, again, the saves are archived but you cannot restore the save games via .gbsa file, only manually. I think it has to do with .xml files.

Edit: As expected, it is indeed the .xml files, for example, on 17 Feb 2019 update, the .xml file reads:
<Path>Game Labs\Ultimate General Civil War\Save</Path>
<RegData>
<Hive></Hive>
<Path></Path>
<Value></Value>
</RegData>
<FileList>
<File checksum="">CampaignBattle\08k6vopo.48s</File>
<File checksum="">CampaignBattle\0anrjexl.t83</File>
<File checksum="">CampaignBattle\0x3r5quu.qma</File>
<File checksum="">CampaignBattle\11ecrkum.56z</File>
<File checksum="">CampaignBattle\15j1lkat.g1w</File>
<File checksum="">CampaignBattle\1raqltxj.kyc</File>
<File checksum="">CampaignBattle\34373ijk.xgs</File>
<File checksum="">CampaignBattle\3kpzdnee.l70</File>
<File checksum="">CampaignBattle\3mcjdrxi.lsd</File>
<File checksum="">CampaignBattle\3qu9jffu.cnz</File>
<File checksum="">CampaignBattle\45kewscx.5v9</File>

But post 23 Feb 2019, the .xml file read:
<Path>Game Labs\Ultimate General Civil War\Save</Path>
<RegData>
<Hive></Hive>
<Path></Path>
<Value></Value>
</RegData>
<FileList>
<File checksum="95D5A30BFBF29D9AA7561CFCB2853482">CampaignBattle\08k6vopo.48s</File>
<File checksum="634887AD23F510C28824DDFAF10BBCF8">CampaignBattle\0anrjexl.t83</File>
<File checksum="3AFA8513CD07C21DDD1ABD1F7192AB3A">CampaignBattle\0x3r5quu.qma</File>
<File checksum="22A28EAFAD67A38E446DF62E08783EFE">CampaignBattle\11ecrkum.56z</File>
<File checksum="EEFED468ED4535083B6C142E32FB4C10">CampaignBattle\15j1lkat.g1w</File>
<File checksum="BC5A02C79CC5D51F8C2AE81033FC22A6">CampaignBattle\1raqltxj.kyc</File>
<File checksum="5F897156FAEDA95FC6F8E8BF9BCECC69">CampaignBattle\34373ijk.xgs</File>
<File checksum="BA2F300F081EAF5838A0590F8C39FF2A">CampaignBattle\3kpzdnee.l70</File>
<File checksum="53FE8C4E7BE3E52AFF7986926081DACB">CampaignBattle\3mcjdrxi.lsd</File>
<File checksum="838427B4677DDAA95E63872456AC5D1C">CampaignBattle\3qu9jffu.cnz</File>

This breaks up the program.

I think it is better to make this opensource, it is a very useful software.