What is inside the gamma ray?

Your question doesn't make any sense.

Gamma ray is not "matter". There is no "inside" or "outside" something which itself is not a material substance. You can only say "what is inside a house?, or what is inside a box? What is inside an atom....etc", but it is of no meaning to ask "what is inside a beam of light?, or what is inside a sound wave?", as light and sound wave are not material substanes.

Gamma ray is pure energy. In the qunatum mechanical aspect, it composes of photons, which are discrete packets of energy as postulated by Einstein when he developed his theory in explaining the photoelectric effect.

The question only makes sense if it is written as "What is gamma ray?". The simple answer to it is "high energy photons".

Will 211 tubes release Gamma Ray ?

Gamma rays are electromagnetic radiation of high frequency (very short wavelength).

There is no atom in gamma rays. But you can say there are photons in gamma rays.

Gamma rays, visible light, infrared, ultraviolet light, etc are electromagnetic waves.