The Stovebolt Site Forums The Shop Area The Electrical Bay Where can I find these dash lights?

Really hard to stay logged in today. Bulbs: the base is known as a BA9, which as Tiny said is a miniature bayonet type. There are two GE numbers that fit that I know of GE-53 1.7W, 1 candle power and GE-57 3W, 2 CP. These are in my spare sockets. The gel is just to color the light green and spread out the light all around. Diffuses and colors. Does not amplify in the sense of making brighter.
The Gels are available at camera stores or on the web. Known as camera diffuser lighting gels. I sent for free samples on the web and used that a few years back.
So.... bulbs: Any FLAP
Socket: Possible FLAP under "gauge lights" or Ebay or wanted forum here.(probably same socket as 55-59 Chevy cluster uses and others)
Gel: As described
Wired: per schematic. Each gauge plate will need a dedicated ground wire to it.
Your GMC looks nice.