The 6AL7GT is a CRT type tuning indicator for accurate tuning of FM or AM radio receivers. The screen is in the top of the envelope and two rectangular green blocks are controlled by three deflection electrodes.
The electrodes are mounted horizontally above the pinch.
The Sylvania name on the base cap.
The screen aperture is viewed through what appears to be a standard GT envelope.
Thanks to Mark Mallison for sending the images.
The wide glass tube envelope is mm in diameter, and excluding the IO base pins is mm tall.
Reference: Datasheet. Type 6AL7GT was first introduced in 1946.