There are 135 smashable security gates throughout the game. They usually signify a shortcut or special area (ie. an abandoned airfield). They have a "Restricted Area" sign on them. Explore every nook and cranny off the main roads to find them.
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Map by ShineDalGarno.
View an interactive guide HERE. There is no way to track gates which have already been found in-game, aside from the total number of gates already discovered. Use the interactive guide to check off gates you've found as you find them to help keep track and avoid some frustration later.
For more help, try using this Video Guide by YouTube user Seigo45.