Beta Gate

The Beta Gate was a second Stargate found on Earth in Antarctica. It was the planet's original Stargate, having been brought to Earth when the Alterans first colonized the planet. It was left behind when Atlantis left for the Pegasus galaxy. It was discovered accidentally by Colonel O'Neill and Captain Carter when an energy blast overloaded the Stargate on P4A-771, redirecting the wormhole from the Alpha Gate to the Beta Gate.

Once removed from Antarctica, it was taken to Area 51 and put in storage. After an accusation of an SG team stealing the Touchstone from PX7-941, Carter discovered the Beta Gate was being used. SG-1 traveled to Area 51 to verify it was still in storage. Upon inspection, they found it had been replaced with a plastic replica. The real Stargate was recovered and returned to Area 51.

Later studies of the Beta Stargate revealed that it may have been one of the oldest Gates in the Milky Way - possibly 50 million years old or more. This could mean that Earth was the first planet to be colonized by the Ancients, though Samantha Carter pointed out that this Gate could have been moved to Earth from elsewhere in the Galaxy.

It remained in storage until SG-1 beamed the Alpha Gate onto Thor's ship to escape before it was destroyed. The Beta Gate was unpacked and setup at the SGC so SG-1 could return home. The SGC continued using this Stargate until Anubis destroyed it with the Stargate destroyer.

The point-of-origin glyph for this Stargate is http://images4.wikia.nocookie.net/stargate/images/thumb/7/72/0eb.svg/120px-0eb.svg.png.

Alternate realities
''In an alternate timeline created by Ba'al, a team uncovered the second Stargate and moved it to McMurdo Air Force Base. However, when Ba'al arrived, Teal'c, who was his First Prime found out about this, and Qetesh ordered him to destroy the base. The Gate was destroyed.''

Behind the scenes
Throughout episodes in which the Beta Gate is installed in the SGC Gate Room (from "Small Victories" to "Redemption, Part 2"), its point of origin symbol is the same as for the Alpha Gate. Presumably, this is not canon, and is instead a production issue caused by reusing the same prop for both Stargates. As shown in "Solitudes", the point of origin for the Beta Gate is different.