Throughout its history CheckPoint firewall changed versions and names, incorporated other products. The last, so far, evolution has been the Gaia operating system released in 2012. All this holds true of course but nevertheless the base platform for the firewall all these years has been Red Hat Enterprise Linux server of different versions. The latest one used for the whole R75 and R77 line of firewalls is based on Red Hat RHEL 5.2 that was first released in 2008. This in part explains why even new firewalls still work on the old kernel 2.18. It doesn’t mean something bad in terms of its security, to remind - 'based on' means even though it is based on RHEL 5.2 it is still heavily secured and stripped down. In their latest communications Checkpoint promise in 1st quarter of 2017 to upgrade Gaia to the kernel version 3.10 as part of the move to Red Hat RHEL 7.
Update 2021: it took Checkpoint time, but on 4th of July 2019 they released R80.30 with the kernel version of 3.10.