Intake Manifold Gasket - DIY?

[NMPastor]

Vehicle: 2001 GMC Yukon XL SLT

Mechanic says throwing off 'lean codes,' intake manifold gasket needs replacing. About $400. (Last time I had a Chevy dealer do this on a Vortec it was in the 700s)

Is this an amateur DIY project (I assume a full day) or do I need to get this one done for me?

Could this be causing P0420/'lazy cat converter' codes as well?

Thanks for the input.