You need to start by looking through your closest matches at your DNA testing company, preferably in the first and second cousin range, for people who are not related to you through your father or your maternal grandmother. Those people will possibly be related to you through your unknown grandfather. If you are lucky enough to find a match at the second cousin level then you will share great grandparents with that match and one of their sons (who is not the grandfather of your match, as you'd then be first cousins or half first cousins) will likely be your grandfather. If you have to go out to third cousins it gets a bit trickier, but the process is still basically the same. Also, the Adoption Angels here at Wikitree might be able to help you with this.
The Leeds Method can determine which autosomal segments come from your different grandparents. I believe it is a matter of determining the shared ancestry of your unknown close autosomal DNA matching relatives. https://www.perplexity.ai/search/e2d69278-c134-487d-8213-cf888c3f1adf (link is good until March 8th, 2024).
He is your mother’s father so you would have also inherited some X-DNA from him.
Hi Julie, you have to use DNA to try and figure out your grandfather. Since this is your mom's dad, Y-DNA isn't going to help. Autosomal DNA is how you will locate him. I used it to figure out my grandmother's father, but it took me several years. I see that both of your parents have passed. Do either of them have any siblings still alive that could/would take a DNA test? It would help to determine whether matches you have are up your mother or your father's side if so. Do you have any half siblings? Testing them would also be helpful if so. If you aren't well versed with genetic genealogy, you definitely will need to locate a search angel to help you. Paul provided the link to the Wikitree people that help. There are numerous Facebook groups also with people that help others. Good luck!
