Thank you for your reply.  I know how to customize my forms using Layout Designer.  I am using an invoice form that I created using Layout Designer that has worked well for me for about 9 years and that used Gill Sans font for some of the text of the invoice.  

The issue is NOT with choosing a font that QuickBooks has listed, but with doing what it is suggesting I do.  When I print an invoice, a pop-up dialog box appears that states, "The form you are about to print specifies the font [Gill Sans MT] which is not installed on this system.  You may wish to install the font.  Click OK to print anyway, or Cancel to skip printing this form."  I used the Gill Sans font in my invoice until I upgraded to QB Desktop Pro 2020.  The new program can't find the font.  

I am using a MacBook Pro with a Parallels system that supports Windows on a Mac, so that I can use Quickbooks for Windows.  The Gill Sans font set is installed on my Mac, and although the operating systems share multiple files, it is possible that I need to also "install" the Gill Sans font set in my Windows system.  I was just checking to make sure that the "system" to which QuickBooks is referring is not just QuickBooks, and that there is a straightforward way to place the font in a location where it can be found by QuickBooks.