is it necessary add snubber network for a SSR made by mosfets?
I do not know what you mean by gate sensitive, but no, that assumption is not correct. 

Excessive dV/dT can cause MOSFETs to misfire. There's also the issue of inductive spikes regardless of what semi-conductor switch you are using.

Part of your false assumption might just be how the devices are used. You find MOSFETs everywhere in both low power, low voltage circuits (where snubbing is usually not required) and only sometimes in high power, high voltage circuits. But you usually only find thyristor in very high power, high voltage circuits (where snubbing is often required)