My grandmother never knew the identity of her father. She gave birth to several children (male and female). I am the daughter of one of her daughters.

I believe I know the identity of her father. She is his only child.

Assuming a sample of his DNA exists, is it possible to prove that I am related to him?

Yes, this is certainly possible. The method used relies upon extrapolation of data so it is not 100% certain and such data has been disputed at court, but there's still a very good chance the analysis would be correct.

At this point I would be satisfied with results that were less than 100% certain. A "positive" test will open doors for me that will eventually lead to documentation of their relationship.

Can you suggest a lab that can handle my testing? I contacted FamilyTreeDNA . They said that their type of testing would not
Help me because I am a female and I have two females between me and mystery GGrandpa.

I am willing to have my son's DNA as well as mine tested.
I can also (eventually) contact a male grand nephew of my mystery GGrandpa and ask him to take a test for additional
Extrapolation.

Lastly, I am operating with the assumption that GGrandpa's DNA is on file in a lab in the USA (I'd rather not go into details on this).

Thanks

I'm afraid I can't recommend a particular lab as to do so would involve a knowledge of their quality control standards which is not something I've had need to undertake previously.

I do know that you need to very careful in your selection of lab as there are a number of dodgy ones in existence. Ask for details of previous work by them in this area.

Data extrapolation from your grand nephew would certainly be useful and if your Grandpa's data is on file in the USA, I don't foresee any major problems in establishing the link.

I wish you the best of luck.