Wasn't the Calamity Gundam developed from the Buster Gundam? What makes it a first-gen? It uses more advanced Trans-Phase armor and has more energy than the first few Gundams. 

The Gundam Wiki even says: 
[quote]One of three [i]second generation[/i] GAT-X machines, Calamity is built on the X100-series frame and inherits Buster's artillery attack concept.[/quote]
That means the Calamity, Raider and Forbidden are all second-gen suits.