Your best bet is to ask your employer for your gross pay amount. You don't want the hassle of possibly trying to explain why the amount you reported on your application is different from what your employer told your background checker.
If you haven't told your employer you're applying, I would either just report net pay or, if you knew how much is withheld from your pay for taxes and other withholdings, estimate your gross pay based on that. As long as you have a valid rationale for how you came up with what you report on the application it shouldn't be a big deal if it does come up during the background check.
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