ARMA 1 is
ARMA: Armed Assault (in USA also known as ARMA: Combat Operations)
ARMA: Queens Gambit (expansion for ARMA)
ARMA: Gold (ARMA 1+QG is the version available on GOG.com, latest released build)

note that ARMA 1 on our back catalogue
http://www.bistudio.com/english/games/back-catalogue

while ARMA 2 and newer is on primary list
http://www.bistudio.com/english/games

ARMA: Cold War Assault is re-release of OFP: Cold War Crisis and OFP: Resistance
(both included) under ARMA branding (latest version 1.99)

ARMA 2: Free is not ARMA 2, it's feature and visual quality stripped version of ARMA 2, compatible with ARMA 2

ARMA 2 engine is on version 1.11, (features, improvements especially netcode from 1.60 aren't included)

ARMA 2: Operation Arrowhead engine is actually at version 1.60, with public betas of 1.61 already released (weekly to bi-weekly)
A2: OA engine has received in 1.60 major netcode and performance improvements
A2: OA is standalone but can read A2 data
A2: OA+A2 data gives ARMA 2: Combined Operations which is what 95% of community uses

DLCs (British Armed Forces and Private Military Company) are only for A2: OA (not for A2)

read this for package details
http://community.bistudio.com/wiki/ARMA2_PackageCombination

ARMA 2: Firing Range is Android, iOS mini-game for mobile devices

i hope that answers some of the questions users got

Any chance to offer ArmA 2/OA on GOG?

I'm hoping once Arma 3 is out they'll gogify Arma 2. I'd buy it here a second time for the convenience of being free of both DVDs and Steam. I won't hold my breath for gogifying Arma 3 for a long while...though, with gog now releasing new games...pretty please, with cherries on top?

Need to worry about upgrading my PC for ArmA 3!

pray harder and especially on this wish http://www.gog.com/en/wishlist/site/automatic_cdkey_system

got the point? :) i hope so :)