Be extremely careful when cleaning the face of a gauge.
The ink used for the numbering is easily "cleaned" off and or smeared on the gauge face.
I suggest just water and a mild dish washing soap and the softest artists paint brush that you can find.
When drying, let the water drip off and then blow dry gently with compressed air. Wiping the face with a soft towel is enough to smear the number ink.
Paul