A few words of caution for you:
SCBA manufacturers indicate that when a valve has been removed, the o-ring is replaced and the valve is then hand-tightened until the valve makes contact with the o-ring. The valve is then torqued to a manufacturer specification (typically somewhere around 75ft lbs). Often this torque specification is printed on the side of your tank. SCBA aluminum walls are more thin than a scuba tank, and over torquing may cause damage to the threads, or damage the resin/glass/CF outer shell. Clearly, this could compromise the integrity of your tank and put people at risk.
Make sure that you use the correct o-ring as specified by your tank manufacturer. They are a very specific size. Although captured, using the wrong o-ring may cause it to extrude during filling, which may be extremely dangerous. The o-ring does not get lubricated, however (depending upon the manufacturer) the valve threads do (very light amount). As stated previously, the lubricant should be a silicone grease free of petroleum products.
If it was me, I would replace the pressure gauge. Clearly it has been damaged, and your cylinder contains a lot of pressure. The potential hazard isn't worth the price of a gauge.