IIRC there are six total gyros on Hubble. Some of them have failed. Earlier this year, one of the three remaining failed as well, so they're down to two. They've dropped to a one-gyro mode now so they won't wear out both at the same time. Once the one they're using fails, they'll switch to the last one.
The two remaining gyros are of a set of "better" gyros that were installed on a servicing mission 15 years ago or so.
They could repair it (as well as give the satellite a much-needed orbit re-boost), but for now they've passed on doing so, presumably for budgetary reasons.
Regardless, completely agree that it's incredible that these spacecraft often outlive their mission plan, sometimes drastically so. Hubble was projected to have 15 useful years (1990-2005), but it's still kicking (though not without the cost of past repair and upgrade missions), and should hopefully last well into the next decade. Longer, presumably, if it becomes compelling enough to spend the money to further service it.