Shadowstalker16: Open General is a FOSS / open version of Panzer General II. Haven't played it yet but its highly regarded and DRM free and also free of cost, so it wouldn't hurt to try it out:
http://www.open-general.com Jorev: PG2 is a good game but the campaign is too short. I would recommend PG1 but for some odd reason it is not available on GoG.
I see, hopefully there are community maps / mods / scenarios to fix that. A lot of older grand strategy and wargames are actually still only available on obscure sites, even if their publishers release their newer games on steam / gog. They probably think old titles are not worth releasing for financial reasons but there are certainly many ''undiscovered'' old ones out there, though their commercial potential is questionable.