How can I change this =GLAMT(F$42,$B88)to showing nothing instead of 0 in my income statement?

Eva:
Go to View | Customize and then choose the "Settings" tab. In the field "Use the following text to replace zero values", press the [Space] bar once and save your change. You will likely have to refresh the statement. Note that this settings applies to all financial statements, not just this one.

" - " (blank dash blank) is often what accountants are looking for.