The DNA test (33 markers) cost about $80 which was a special offer by Ancestry.com. Although it is not cheap you can find out where your ancestors came from after the initial exodus from Africa. Secondly, and this appeals to family historians more, you can match your results against those in the various indexes that can be searched on the internet. If you find a match over say, 30 markers, then you most likely have found a relative with a common ancestor no more than 100 years ago. What is more, you can believe the result, which is more than you can sometimes say matching Parish Register records!
Concerning Judy Garland's ancestry, I have read David Shipman's biography with the genealogy done by Virginia Wilkinson. She says that Gumms were first recorded in America in 1782 in Sussex County, Delaware. Whether she means Gums as well I don't know but that certainly doesn't tie up with what I have found. Frank Gumm's grandfather, John Alexander Gumm I believe was from Tennessee and I have found nothing more. perhaps you can take it back further from your researches. Although Ethel, JG's mother,had an Irish mother and Scottish father nothing is sais about Frank's possible roots. Finally, I can say that in the UK, the form of name is nearly always Gumm unlike the USA. originally it existed as Goom, Goome and Gum but in Victorian times it settled down to Gumm. If Franks ancestors came from the UK then the two m's would be most likely. Any views on all this?