Hello, I hope you are doing well

No, there is no such benefit for grandchild of a service person. You can get relationship certificate in respect of only those relatives whose entry has been made in service records of the service person. And this  record is generally restricted to parents, spouse, siblings, progeny and any direct dependents.

So, since your father was living at the time when your grandfather was in service, your name could not have been entered in your grandfather's service record and because of that reason you wouldn't be able to get the relationship certificate.

Hope this helps