What is the gravity field of a photon?

Some time ago I asked research community about gravity of photons. Question was formulated as follows:

Let us imagine hollow sphere having mass M and with ideal mirror surface of inner walls. Equal quantities of antimatter and matter with mass m are placed inside sphere. Total mass of the system is equal to M+2m and the system generates gravity field corresponding to the total mass.

What gravity field of the system will be if matter and antimatter will annihilate and resulting photons will stay inside sphere?

Discussion was very interesting, but I could not received unambiguous answer. You can find discussion in site of ResearchGate:

https://www.researchgate.net/post/What_is_the_gravity_field_of_a_photon

Do you have answer?