The '68 GCA prohibited felons from owning guns. It's based on the same loss of right, that took away their vote. It was an act of a legislature. Now htat you've looked up expost facto, go for "attainder".
"If the government has a power, it has it in all cases whatsoever. "
See #47. It only applies to legislation limiting the rights of felons. It has noting to do, with imposing additional punishments, or passing laws with would legislatively add to their crimes.
OK. So long as the additional restriction is a "civil" restriction, and not a criminal penalty, you got me.
It raises the question: at what point is a "civil" penalty so onerous that it can't be justified?
Under the scheme you outline, it would appear to be allowable for the state to enforce even, say, speech restrictions on convicted felons, forcing them to subject all their public utterances to prior restraint.
Is that correct?