It certainly is old, your Great Uncle had good taste and that's a very nice all original solid gold Omega Constellation. The band isn't original and should be removed as the spring loaded end links will chew holes in the solid gold lugs.
If you intend to sell it, do nothing to it except take good clear photographs and let the market find the value, I'm sure it will do that without any problem.
You may not have to worry about that though, just keep an eye on the little envelope at the top right of your screen.
PS: Where are you located?
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