Gateways works just like in SC:BW with one notable exception, they can be transformed into Warp Gates.
The protoss' ability to manipulate time and space far surpasses the limited understanding of other races. A gateway is a structure capable of opening a spatial rift via which protoss forces can be warped directly from their home territories to the battlefield. A phenomenal amount of energy is required for this operation so, as with all protoss structures, a gateway can only function if it is powered by the psionic matrix formed by a network of pylons or phase prisms.
Once a gateway has been established, it can be reconfigured to an even more remarkable device: a warp gate. The technology used in a warp gate is barely understood even by the advanced sciences of the protoss, having been reverse engineered from ancient Xel’Naga devices found on Aiur and Shakuras. It is a measure of the desperation of the protoss that they are now willing to attempt to build such devices themselves.
Functionally a warp gate is similar to a gateway in that both devices open a spatial rift. However, protoss forces moving through a gateway must emerge in close proximity to the structure, whereas those summoned via a warp gate can be projected to any part of the battlefield that lies within the psionic matrix.
When used in combination with the mobile phase prisms, warp gates allow the protoss unprecedented flexibility in their force deployment. Warp gates can transport protoss warriors directly to the front lines, send defenders to a beleaguered outpost, or build up attack groups in unexpected sectors. Enemy commanders must be constantly on the lookout for phase prisms or they can quickly find an entire protoss army deployed on their doorstep.