See the GDL Reference Guide (Help menu - which is a link to the program's Documentation subfolder), Global Variables: SYMB_MIRRORED on page 337 (US AC 22 version of PDF).
You can perform an IF on this value to determine if the user has mirrored the object, and then 'un-mirror' it ... or perhaps just 'un-mirror' the text. If you haven't handled rotation and your object text yet ... SYMB_ROTANGLE will give you that, so that you can re-orient the text. Etc.
As an aside... since you're digging into GDL... many things don't show up in an obvious place in the Index at the back of the manual... e.g., no entry for Mirror. Instead, search the PDF with the Adobe Reader/Acrobat "Find" function for what you're looking for.
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