

Great grandfather’s sister’s grandson is your second cousin once removed. That guy is the second cousin of one of your parents because they share great grandparents with one of your parents. A grandparent’s sibling is a great aunt or great uncle to you. A great grandparent’s sibling is a great great uncle or great great aunt to you.
This map is apparently from Wikipedia, and is based on a 1995 CIA ethnolinguistic map. The version here is identified as from 2007, with some changes made by wikipedia commons editors (visible changes in Abkhazia and other places). The changes are marked as disputed and unsourced, and it does seem like someone took the letters marking general presence of an ethnolinguistic group (indicating sporadic presence) from the 1995 map and converted those letters into solid bands. This does seem to be a dubious change, as I cannot find any support for this solid separation of groups in Abkhazia and elsewhere.