I would look on Craigslist in his area for similar pieces to find what people are asking for them. In my area, they are not in high demand of that style and era.

There is some Art Deco decoration. . But, the finish itself, looks more 60’s. How long has Grand Dad has this piece?

I am as far from an expert as you can get, but I have questions and ideas. When did they start using glass shelves? I don't know. IIRC they were not using curved glass very much anymore in the 60s and 70s, and they were not doing the lovely circular design on the large glass areas. OTOH, when did they start making this combined china cabinet on the top, buffet/sideboard on the bottom?

The style of the hinges on the glass doors makes me think the 70's or 80's. To me it looks like a higher end china hutch, perhaps European import. Maybe Italy, Romania or such? Romanian furniture from that era often was quite elaborate.

A furniture store in FL offers an Italian china hutch in a similar style on its website:

Modern Miami: Milady Lacquer China Cabinet Made in Italy

Wow, good find...it does look very similar.