I am currently working on a artbook and the printer asked to update to a 20 mm gutter bleed and I'm not sure how that should be set up in my file. I also need help with where I should place the full art spreads with that new bleed set up.

Hi ​@KoolaidGirl17 , It sounds like you might be confusing gutter and bleed? Gutter normally refers to the page’s inside margin:

While I agree with ​@Dave Creamer of IDEAS in general, that inside bleed is HUGE, which makes me think this must be a pretty thick book and the pages are not going to lie flat, and a good bit of information at the gutter is either going to be duplicated on both pages, or is going to be buried in the binding, which could make your crossover spreads look odd, no matter where in the book you place them.

I would recommend you have a talk with the printer about this... 

Did you set up the bleed in your document?

If so, you have three possible spreads:

1. Spreads with neither page has anything touching the gutter.
2. Spreads with image/object stretching across both pages. 
3. Spreads with image/object touching gutter on only one page of spread.

You only need to deal with the gutter on the third option. 

1. To do that, select the spread on the Pages panel.
2. Select Selected Spread to Shuffle Pages option to deselect it. 
3. Deselect the spreads and select the right page only.
4. Drag it to the right until you see a vertical line appear and let go.

That will separate the spread so when you output as a PDF (as single pages), the bleed will be correct.