You can GRAY and HIDE additional text by referring to it in the dialog script in the same format that it appears in the Dialog Contents box in the Dialog Builder, which I understand to be "{" followed by the first 44 characters (and then a number if necessary to distinguish it from other additional text in the component file). See Computation #0036: HIDE/GRAY Additional Text.
Computation buttons are additional text, but include colons, which cannot be used in a component name and therefore, according to the manual, make the additional text ineligible for use with GRAY, HIDE, etc.
I have found that I can GRAY and HIDE computation buttons (and some other ineligible additional text) if I quote the reference. For example, to hide my computation button @COMPUTE:cmpComputeZipCode:Insert Zip Code (See Help), I would use this line in the dialog script:
HIDE "{@COMPUTE:cmpComputeZipCode:Insert Zip Code ("