My Grandads old Watch, any info would be appreciated.

Posts
31,026
Likes
36,425
I'm afraid that's a counterfeit
 
Posts
4,437
Likes
18,230
Yes. It is a fake and holds nothing but potential sentimental value.
"Autobahn Gold" it was called. The case is 99.9% not gold either (even if stamped like it). In fact Omega never made such marks on the outside of a watchcase (only inside of the case back).
Take it to a jeweler if you like to have the metal checked for gold content but I assume it is very near nothing.