Basics of GEDmatch:
Run a one-to-many report for your “kit”. Everyone who uploads their raw DNA files to gedmatch gets allocated a kit number (starts with A if it was from Ancestry T if from ftDNA, and M if from 23andMe)
When you run the report the system goes off and matches you against all the other people in the database. It will return a page of 2000 matches, sorted by closeness of predicted relationship (closest matches at the top).
To do further matching and get more details on how the match is made, run a one to one report using your kit number and the matches kit number. At the bottom of your one to one report you will see an “mrca” number. Mrca means most recent common ancestor. It will give a number, e.g. 3. This means that based on the amount of shared DNA the system predicts your most recent common ancestor with your match is 3 generations back. Count your parent gen as 1, then grandparents 2, so 3 means that system predicts you share great grandparents (based on the amount of shared DNA. – there can of course be other relationships that share about the same amount, so it’s just a prediction based on numbers, to work out the exact relationship you need to build a tree)
Shared DNA is calculated in an amount of cM (centimorgans). The more cM you share the closer the relationship is. Small amounts of cM shared may just be a coincidence, so stick to looking at matches with a high number of matching shared DNA.
Gedmatch allows you to see where you match someone, exactly which chromosome . This is helpful when trying to find a lot of matches all in common (if you all match each other on the same chromosome at the same location then you all have the same common ancestor). But bear in mind you have two of each chromosome (one from dad, one from mum), so ALWAYS check your matches using the one to one report to check that each match , matches each other before you assume they are all from a common ancestor (some will be a common ancestor on the maternal chromosome and some will be from a common ancestor on the paternal chromosome)
See more here: