Can you post a straight on shot of the dial under more natural light, just hard to tell from that pic if its a gold dial or one of the painted ones due to the blue-ish reflection in the crystal.

 

He's not saying its fake, he's saying its a redial so the certificate has no bearing on that. Because of the poor water resistance of watches in that era it was a common practice during servicing for the dial to be repainted, unfortunately that lowers the value of the watch significantly but as an heirloom for you to keep and wear that matters rather little, its still your grandfather's watch, as he wore it.

 

The glass is plastic and quite cheap, they're typically replaced at each service as its a wear and tear item, but its the paint on the dial that has been replaced. The old would have been bad and water damaged so they'd strip it back and repaint it which is why it looks different but as @TexOmega says wear it proudly it was your grandfather's and its an authentic gold Omega.

 

Yes its authentic as in real, just not original as it left the factory due to the redial.

 

Just as a visual aid based on your username, this is an authentic and real Porsche 930, but due to the bodywork (among other things) being re-done by someone else its not original as it left the factory anymore.