The Dimensional Gates are referred to a number of times throughout the MK literature. One notable instance is the MK3 background story: "This unholy act gives Shao Kahn to step through the Dimensional Gates and reclaim his Queen. Thus enabling him to finally seize the Earth Realm..." Also, in the MKII comic book Shang Tsung says, "Any realm can exercise its right to challenge as long as it can breach the dimensional gates that separate it from the other realm." A "breach" is, in reality, an imbalance in the Furies. In the MK3 background story found in the MK3 pocket kodes by BradyGames, the Furies are described as, “negative and positive forces that keep our unstable universe from collapsing into itself.” So it stands to reason that the Furies, are the energies of which the Dimensional Gates are made.
I've found in my research that these "breaches" Tsung refers to can be divided into 3 levels of severity.
A relatively small, localized, time-sensitive, and volatile breach usually manifests as a portal. Portals often allow for only a few beings to pass from one realm to another and are typically only open for a short time.
A realm merger is a larger scale weakening of the Dimensional Gates between two realms. When two realms become one, the Gates separating them dissolve. This process is only semi-permanent because if the realms are un-merged, the Gates reappear, fully intact.
A large-scale weakening of the Dimensional Gates would involve the Gates separating several realms. Fusing the Kamidogu, for example, would cause all the realms to merge together and thus dissolve all the Dimensional Gates. Such a thing would mean the end of all Reality and stands as the greatest fear of the Elder gods.