Did you try removing the gage and bolts and cleaning everything properly? Any dust in the connection can cause creaking. My seat used to squeak mercilessly all the time no matter how much I tightened the bolts, but eventually, I took everything apart and cleaned all the connecting parts and it did the trick. Hasn't squeaked in years.

You could try grease, but that might make road dust stick in the bolts and bosses and making things worse.