The flag to which you refer, I believe, is the pinnafore (triangular) flag on the right and not the country of origin flag on the left.
It represents players who have accepted membership in WGT Nation, and it is by invitation only.
Here is WGT's blrub on the subject:
WGT players who are either Legends or Level 90 and above are eligible to join. Additionally, a limited number of other top players will be invited to join the program at the discretion of WGT.