Powerup Gates are a new mechanic featured in Spyro 2: Ripto's Rage! Thanks to them you can, i.e. fly or make really high jumps. Spyro can use these gates to reach places that can't be reached by using your regular abilities. However, these gates aren't active right from the beginning. You need to activate them first before they can be used by Spyro. Once you activate them, you will be able to use these gates unlimited amount of times.
In Spyro 2: Ripto's Rage!, every defeated enemy drops a spirit particle. The game keeps track of how many particles you have. Every Powerup Gate requires a certain amount of spirit particles. Get enough of them, and you will activate a given gate. You can check the requirements by approaching an inactive gate.
Spirit particles aren't carried over from one level to another. Spyro starts with zero particles in every level. If you don't have enough particles, try to go back to the previous parts of a given level, and look for more enemies.