Looking at that image above, the popularity is 164.
Popularity is a determinant on how successful a club is , and it expressively records ; wins, loses and investment capability of a club.
Popularity is unique and important factor in GT. Popularity also determines income because as the popularity becomes higher so as the income.
Investment like in funding account with some certain amount of euro adds popularity score. By that way GT encourages investment.
More popularity more fans and more fans more money to the club. Number of supporters in each game is determine by popularities of both home and away team.
Each time team loses a game the popularity reduces by 1%, each win records a 1% increase in popularity. Also remember, having a bad stadium, less number of seats for fans and a GS lower than current POP reduces Popularity.
Finally if you increase your ticket price your popularity reduces by 1.5%.