Hi,
Check Point use ISC (Internet Systems Consortium) DHCPD which is the same as you find on other Linux machines forexample. So if you have some experience by manually editing dhcpd.conf file you can go ahead and do that on your gateway(s).
You can open the dhcpd.conf file in vi by running this command in expert mode on your gateway: vi /etc/dhcpd.conf
Add your option inside this configuration file.
Remember to run save config after you are done with editing.
Vi commands: (since I always Google them):
Esc followed by:
:q (to quit)
:q! (quit without saving)
:wq (write and quit)