I'm preparing my G20 engine for assembly right now. How seriously do I need to clean off the old head gasket?

I removed all the old material, there is just some darker color remaining here and there, but it looks smooth and flat so far.

 

Just because it looks 'smooth and flat' doesn't mean it is.

If it's a nice flat sealing surface then no problem, but if you can actually feel any of the residue, then you need to clean it some more.

 

Yeah thats been the idea so far. I can't feel any material, but im a noob at this sort of work so I dont have any instinct for it. I'll get some measurement tools and quantify the flatness.

 

If it dosnt feel like sliding ur finger over glas then its not clean enough. just dont scrape it with metal or a blade. try scraping with something plastic if needed. get brake parts cleaner on a soft cloth, spray the cloth and whipe.

 

Get a metal straightedge and lay it across the block.
if you have a piece of square metal tubing, take it and wrap it in some 400+ grit sandpaper and run it across the top of the block. you'll find the high spots.