Hi,
I did not find a snmp response matching the actual installed version number. CheckPoint MIBs did not mention that too... Check Point SNMP MIB files  - at least I didn´t find any, that sounds promising.
via api command "show ips-status" you get some information. 
[Expert@somehostname:0]# mgmt_cli show ips-status  --format json
{
  "last-updated" : {
    "posix" : 1532076480000,
    "iso-8601" : "2018-07-20T10:48+0200"
  },
  "installed-version" : "635184761",
  "installed-version-creation-time" : {
    "posix" : 1531900800000,
    "iso-8601" : "2018-07-18T10:00+0200"
  },
  "update-available" : false,
  "latest-version" : "635184761",
  "latest-version-creation-time" : {
    "posix" : 1531900800000,
    "iso-8601" : "2018-07-18T10:00+0200"
  }
}
Assuming the management is able to download updates on its own, you´d see it on update available.
Or you can compare "latest-version" with "installed version". 
Depending on your monitoring solution you can implement some kind of a script there, that compares the outputs - i.e. with the actual time - and notify, if no update has been done for x days, or a update is pending.
Hope that helps
Regards,
Daniel