I started using GEDMatch a few years ago after I started testing my DNA and wanting to find more cousins and learn how to better analyze my results. Genetic genealogy is an important part of the genealogy tool kit to verify your paper trails. And sometimes you uncover a surprise like an NPE, a non-paternity event (ex. a parent isn't a match to a child). In my case, you find out about a child that you never knew was in the family. Check out my blog post about Thomas Corcoran.
The process to add your DNA profile is fairly simple. You can register for a free account (or pay $10/month to be a Tier 1 subscriber for some tools). You download your DNA file from the site (usually a commercial one like Ancestry, 23andme, FamilyTree DNA, MyHeritage, etc.) and upload it. Note: GEDMatch has created a generic upload that will work for other kits as well. Then you usually wait a couple of days before you can run the one to many matches to see the DNA cousins you have. Suffice to say I have a lot of DNA matches b/c of my Ashkenazi Jewish ancestry.
You can also add a GEDCOM file, which contains information from your family tree. This can be useful in conjunction with your DNA profile to search across your DNA matches and help narrow down cousins and hopefully find your most recent common ancestor (the one you both have in common).
GEDMatch has been in the news recently with a new suspect arrested in the "Golden Gate Killer" case. Law enforcement used GEDMatch to conduct familial DNA testing, where they locate relatives of the unknown DNA sample found at one of the murder crime scenes. This helps narrow the suspect fool to a specific family. The officers then constructed a very large family tree based on DNA matches' trees (possibly their GEDCOM files that were uploaded) to narrow down potential suspects. When a suspect was identified that lived miles of many of the murders, the police put the suspect on surveillance, gathered evidence that had his DNA (ex. like disposed trash), and confirmed it was a match. A second DNA sample was taken from the suspect and he was arrested the following day after the match was confirmed. 1
When news spread of the arrest, discussions started occurring within the genealogy community in regard to how the police conducted their investigation to identify the suspect based on DNA. One of the main concerns was that law enforcement used GEDMatch without a warrant and that in doing so violated the rights of users who upload their DNA to GEDMatch for genealogy reasons and probably had no idea their information would be used in a police investigation. I think it's going to be an interesting trial as we learn more about the investigation.
GEDMatch's terms of services don't prohibit use of the site by law enforcement and warn users that the site could be used for "other purposes". 2 Now is the time to make terms of service crystal clear on what private web sites can be used for so that users can understand any potential risks of having their DNA uploaded, whether it's an unknown relative or a possible criminal investigation into someone related to you. I do want to encourage everyone not to panic and purge your data from GEDMatch. DNA is being used more frequently to clear suspects of charges and it's in our benefit to assist law enforcement where possible to solve these horrible crimes. We do need to make sure that our laws are up to date as technology is evolving to make sure that any necessary procedures are in place (ex. search warrants) before police start searching in a pool like GEDMatch (especially if they are not versed in constructing family trees like us genealogists).
I'm hopeful that with these changes, we can continue to capture the bad guys and help guarantee a conviction. It would be very sad if police went to such lengths to identify a killer and the case is dismissed due to improper handling of evidence.
1 - https://www.vox.com/2018/4/27/17290288/golden-state-killer-joseph-james-deangelo-dna-profile-match
2 - See 4/27/2018 announcement on gedmatch.com