Code p0420 reads on scanner

P0420 reads on my 2002 Pontiac grandam V6. I recently had a head gasket job done (and Egr valve and thermostat and catalytic converter put on before the head gasket)I am trying to pass emission but my engine light keep coming on.Last week I replaced my EGR trying to get engine light to stay off so I can get my pass my emission.

Nice site :-) thanks!