For what? Island bombardment? Sinking civilian ships? Evading the incoming missiles? Because honestly - I can't figure out why a CG cutter would need this kind of armament aside from 'feel good' component.
JCG PLH-31 and 32 class has two 20mm M61s, as well as two double barreled oerlikon 35mm guns, or two 40mm bofors Mk3s respectively.
Hamilton class had one 76mm oto melara and even a 20mm phalanx ciws
Legend class has a 57mm and a 20mm phalanx ciws
And remember all these ships are smaller than the 12k ton cutter... So if anything, by displacement, this particular cutter is actually somewhat underarmed.
As for the role of such weapons, it's basically warning and enforcement against relatively soft surface targets.
A medium calibre gun and autocannons are pretty standard fit. I am a little confused as to why the USCG cutters have phalanxes given it is very much an anti air and anti missile last ditch defence but they do also have an anti surface function.
I also don't think PLAN are producing 100mms anymore...