Cake decoration gels

Sodium.citrate is likely used as Buffer..just like in gelled desserts where gums and acidulants are present .

Is it necessary to add sodium citrate for gelling?

It's a stabilizer, as too much acid will have destabilizing effect on the gel structure.

What is the gelling agent you are using to gel the colourants in.?

Usually pectin..

Guess its not just sodium citrate that is added, because sodium citrate is helping to stabilize the pH. What are the other gelling agents present in your product. And structure is also related to how much of each agents (or combination of other acids + salts) that are used in this preparation.