Is there harm in using a GFCI outlet as a switch?
Certain GFCI+receptacle devices are rated for this purpose.

Use one of those.  

There are GFCI devices whose buttons are specifically labeled Off/Test and On/Reset.  The reason the "off" and "on" are there is because these devices are built and rated to be on/off switches using the GFCI function.  The difference between these and regular GFCIs is these are made for many, many cycles.  However, these are difficult to find (especially on the Web, where plain GFCI results bury them) and you will probably need the assistance of an electrical supply house to locate them.  One example is a Leviton 8590-RB, however that's a deadfront and you'd want one with sockets.  Leviton calls it a switch-rated GFCI.   


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If you're fitting a deadfront GFCI to gain this functionality, just mount a box next to the receptacles and put the GFCI there, using the LOAD terminals on the deadfront to feed the receptacle(s). 

Far easier to find is the GFCI+receptacle w/ switch device. 


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Typically on these, the switch is two pigtails.  The GFCI+receptacle uses all 4 terminals (LINE hot/neutral LOAD hot/neutral) plus ground, of course.  In this case you would not use Test/Reset to turn the load on and off, you'd use the switch.  The switch and its load can be placed on the LOAD side of the GFCI protection (carefully).