The only OS that matters anymore is Gaia, which is effectively a combination of the best features of SecurePlatform and IPSO.
Now for a brief history lesson 🙂
IPSO was created by a company called Ipsilon Networks that Nokia acquired at the end of 1997.
As I recall, IPSO means "IP Switching Operating System" and was, prior to its use for Check Point, used for ATM Switching (the interface type, not the machines you pull money out of).
It basically became a purpose-built operating system to run Check Point FireWall-1 and was also used by Nokia Networks for various functions.
IPSO was based on FreeBSD and was used on the various Nokia appliances, as well as the Check Point-branded IP Appliances.
SecurePlatform (a.k.a. SPLAT) was Check Point's dedicated operating system for Security Gateways and Management that was created in the 2000s.
It is based on RedHat Linux and ran on generic hardware, initially.
Later, it would also run on specific purpose-built appliances.
When Check Point acquired Nokia's Security Appliance business back in 2009, the best elements of both SecurePlatform and IPSO were combined and Gaia was created.
The first version of Gaia shipped with R75.40 and is the only OS that is used today.
SPLAT and IPSO ended at R77.30 and are now end of life.