you know your grandfather's name (do you know where he lived).
You can join ancestry.com and do a search for Jack Gaskill
do you know your father's name?you could do a search on him also.
look for your Jack Gaskill in the 1930, 1920 censusyou should be able to find him and who he was living with. (this will be clue to your great grandfather. If you know the birth year of your grandfather Jack and location you should get some hits.
I would be willing to do this initial search for you need the following to start Jack Gaskillbornca?and location (fathers first name if known)should be on your birth certificate