Greetings to everyone!

- GOG's Alien Shooter is out of date! Steam version is the best, as it received bug fixes among other cool things...
- Alien Shooter 2 from GOG and Steam are the same version;
- Zombie Shooter from GOG and Steam are the same version;
- GOG's Zombie Shooter 2 is out of date! Steam version received a full HD resolution update, and apparently some bug fixes here and there.

I can't say anything about the ZOOM Platform versions, because I didn't test them. And I also haven't tested the other games in the series, so I don't know if there are differences between Steam and ZOOM. If anyone has tested these versions, let us know about any differences. =)

Ladies and gentlemen, here are the references:

Alien Shooter (GOG): https://www.gogdb.org/product/1207658898#builds
Alien Shooter (Steam): https://steamdb.info/app/33100/patchnotes/
Alien Shooter 2 (GOG): https://www.gogdb.org/product/1207658711#builds
Alien Shooter 2 (Steam): https://steamdb.info/app/33120/patchnotes/
Zombie Shooter (GOG): https://www.gogdb.org/product/1207660363#builds
Zombie Shooter (Steam): https://steamdb.info/app/33130/patchnotes/
Zombie Shooter 2 (GOG): https://www.gogdb.org/product/1207660503#builds
Zombie Shooter 2 (Steam): https://steamdb.info/app/33180/patchnotes/

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Note: Zombie Shooter on Steam is from 2016 and on GOG from 2018, however, they are the same version 1.2.
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I hope some GOG moderator/curator sees this and contacts the Sigma Team so these games can be updated.
GOG customers DESERVE THIS ATTENTION!

Thank you and best regards! ;)