Guys,

Do you consider gusset support for small bore/branches in CEASAR II input? If yes, how and what kind of restraint is it? Thank you in advance.

from my experience, we don't consider it as a support in cII.

Is this gusset strengthening the joint between the header and branch?
If so, this is not a restraint to "ground", i.e., a system boundary condition, as you are not controlling the absolute position of a node but you are controlling the relative position of one node to another.
If you wish, you could improve the model by increasing stiffness but I doubt there would be much value in modeling this gusset. (Certainly, if you have a large header OD and a small branch OD, running a rigid element from center of header to wall or the use of an offset should be considered.)
Adding gussets to small bore connections may reduce fatigue failures at those connections but I see it as a design consideration that does not find its way into pipe stress evaluation.

rileykyrra and Dave Diehl,

Thank you for your reply. We have common understanding and it helped me to clarify things in my mind.