1. in a new layer draw a circle selection using the ellipse selection Tool and fill it in white, using the bucket tool or the brush.
(For this, you can recover the selection from the path you saved before, or try to make a fresh new selection th same size the circle of the gauge).
2. Deselect pressing CTRL+SHIFT+A
3. With the Ellipse tool draw again a circle selection BUT now try to leave a piece of the white circle unselected (this will be the moon), play with the selection size in order to get the moon shape unselected, and the rest of the white circle must be inside your selection (Try the make a bigger selection than the gauge circle and move it to the right side of the gauge circle)
3. Once you got the moon unselected just erase your selection (the rest "not moon" white circle). THIS IS THE SMALLER MOON

For the BIGGER MOON...

4. You need to repeat the earlier steps, and try to make a thicker moon, like three or four times the thick of the small moon.
Once you have it...

4.1 apply a gaussian blur filter, (i previously colorized the moon just for a better look in the tutorial for this step, but it's not neccesary).
4.2. Recover the selection from the path and inverse it, [menu select>inverse] (or do the same thing of step 1) this is for erase the external border of the bigger moon
4.3. Erase this selection for fit the blurred moon inside the gauge circle.

5*. Use the Scale tool (Shift+T) for fit both moon layers to the gauge circle in case you need a better fit of them,
*BUT if you did it right, you don't have to do this step

6. Just adjust both layers opacity and you are done.


I hope you find it useful.

Step 1: Make the circle selection using the Ellipse tool (E) and pressing Shift for getting a perfect circle. (don't forget to save it as a path).
Step 2: Create a new transparent layer [Menu Layer>New] OR typing Ctrl+Shift+N OR in the Layer Dock (You get it pressing Ctrl+L) just clik in the New Layer Button.
Step 3: Draw a Blend with the Blend tool (L) Gradient FG to BG, Linear shape and select 2 shades of gray: one medium gray and the other almost white.
Step 4: Now go to [Menu Select>Shrink] and type like 25 or 30 px and accept. Your selection will shrink and let you a metal border outside.
Step 5: Draw the blend again but in opposite direction.
Step 6: Skrink again the selection but now use a minor pixel value.
Step 7: Erase the selection pressing Delete or with the Eraser tool (shift+E)