When you are appraising the fee simple interest of a subject property your GRM should be based off of market rents of the comparables, not actual rents. A lot of times they are the same in my area.
If you know the market rents of the subject and the two are identicial, or very similar, then you should be able to reasonably estimate the market rent of the comp.
Use the market rent.
I use market rent for all my comparables and discuss the difference of market and actual rents in my sales comparison approach comments, if they are different.