Ex post facto, baby. Ya gotta love it.
It's not ex post facto because it doesn't criminalize conduct prior to it's enactment. It criminalized post enactment possession of firearms by certain persons.
You can't criminalize or increase the punishment for an act I did today by changing the law tommorow but you can change the law today to criminalize or increase the punishment for an act I might do tommorow.
I don't agree with the lautenberg amendment but it is not an ex post facto law.
In this case you are generally tried under the laws in effect at the time the offense was committed.
The GCA excludes misdemeanors punishable by two years or less. I would argue that the GCA imposes on the government a requirement to prove the offense he was convicted of was punishable by more than 2 years in prison at the time he was convicted.
This case is completely different from Lautenberg as the amendment changed the definition of a qualifying crime. In this case the state changed the maximum punishment for the offense after the fact thus the feds considered it a qualifying offense.