515 is the max gear score you can attain in the Division 2 WITHOUT the WoNY expansion. so if you don't purchase the WoNY expansion then the highest you can go will be a gear score of 515.
Once you purchase and PLAY the WoNY expansion you start with a new leveling system with a max level of 40 for WoNY.
Once you complete WoNY the "level system" is replaced with the SHD tech level which applies in both DC and WoNY.
Hope that helps.