Gate lights were invented as an extra warning to the 8" and 12" lights for visual aides. The light on the far gate would always stay lit, while the other two lights would flash along with the 8" or 12" lights. Rarely sometimes do you get a far light that's supposed to be on flash. I've only seen this happen at two crossings in my lifetime. Four-inch lights have been around for a long time. In the early days, the gates would bend off of the gates, altough they weren't 4". I don't know the exact size of those older lights. Some gates like this are still seen in other countries around the world.. The 7" type started replacing the 4" to provide better visibility, especially at night. Since L.E.D. lights are not made for 7" lights, the 4" lights are making a big comeback. A lot of 7" lights in the Arizona area have been replaced with 4" lights, or Aurora lights as seen below, and the 7" gate lights are getting rarer to see anymore.
Picture
taken from Carson, CA
4" gate lights.
These lights followed after the older type that extended off the gates back in the 50s. These small lights are still in service today.
One popular seller of 4" gate lights is RECO.
4" gate lights are used in Argentina (when they're not stolen, which is quite frequently). The only difference is the front half of the 4" is red, while the rear half is orange!
Picture
taken from Skull Valley, AZ
7" gate lights.
Pretty much like the 4" lights, but a little bigger. These are a little hard to find, but they do exist in a lot of places. *NOTE: Safetran does not make 7" gate lights anymore.
General Signals, WCH and US&S are manufacturers that I know of that still sell 7" gate lights.
Picture taken from General Signals Website
Here are comparisons on what the 4" and 7" gate lights look like. The outer lights are 7", while the center light is 4".
Pictures taken from Williams, AZ and from the Aurora website.
Alstom/Aurora Gate arm L.E.Ds.
This is one type of L.E.D. light that you can choose from. It isn't a popular light with some of my regulars.
Pictures taken from the WCH website, and from East Lansing, MI.
WCH
Gate arm L.E.Ds.
Here's another type of L.E.D. light to choose from, made by Western Cullen Hayes. I have personally seen these, but never in action (in person).
Picture taken from General Signals Website
There are L.E.Ds that you can insert into the existing 4" gate lights made by General Signals for $39.99. I saw lights like these activate at the U.S. 24 crossing in Gilman, IL. Currently, there are no L.E.D. lights available for the 7" light. All three types of L.E.D. gate lights don't fade in and out when flashing, and I'm not aware of any fading L.E.D. light for gate lights at this time.