I'm going to change the head gasket on my 2004 honda civic what do I need?

Do I need the whole gasket kit or can I just buy the head gasket itself

Reason the do a kit is it's never simply a head gasket needing replacing. Disturbing other components is needed and they too use gaskets.
You need torque wrench and workshop manual for correct dismantle and reassemble with torque stages and specs.

Don't try to replace a head gasket if you never have done one before, you will just have to play twice as much to get it done correctly. No insult but there are so many thing which can go wrong. Like having the head checked out and machined, then the re-torquing sequence must be done perfectly.