With the gauge removed from the body you can now carefully use a flat blade screw driver, or a small hook, to remove the black o-ring from the brass stem on the gauge.

Match the size of this o-ring with one from your kit. Likely, the new o-ring will have a slightly thicker "section" or thickness but should have the same inner diameter. In this application the best choice was a 2mm section, 4.4mm inner diamater o-ring.

Rub a small amount of synthetic, preferably silicone based grease on the o-ring.

Set aside the gauge until reassembly.