I have a GFCI outlet in my garage which is about 4 feet off the ground. I have a couple work lights plugged into it which can only be turned on and off by plugging/unplugging them. I have recently just been using the "Test" and "Reset" buttons on the GFCI outlet to turn the lights on and off. It's a lot easier than unplugging both plugs and later having to bend down to pick them up off the floor and plug them in again.
Can doing this a couple times a day do any damage to the GFCI outlet or to anything plugged in to it?
Certain GFCI+receptacle devices are rated for this purpose.
Use one of those.
There are GFCI devices whose buttons are specifically labeledOff/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.
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).
Also note that using a GFCI test button as a switch is very different from using an ordinary circuit breaker as a switch. As I understand it, while using an ordinary circuit breaker as an ordinary on/off switch is often not recommended (and in some situations may even be a code violation), there are at least some circuit breakers which ARE designed to be fully capable of frequent switching use. A GFCI test button is quite different - it actually produces a fault equivalent to a dangerous situation (but within the GCFI module so that it is safe to do so) in order to trigger the GFCI circuit. The result if everything works as expected is that the outlet is turned off. But that is a side-effect and NOT the direct result of pressing the test button.
As noted in another answer, there are some GFCI devices that either include an separate On/Off switch or are switch-rated. Those are certainly options as well, though they may be harder to find (i.e., may require going to an electrical supply house rather than Amazon or Home Depot or Lowes).
The TEST and RESET bottons on a GFCI outlet are not designed for use as a light switch or load switch. Constant usage as such will stress it's internal mechanism such that it may not function properly when called upon actually in it's ground fault trip mode.
If you want to see for yourself then take apart a GFCI outlet and a light switch and see how much more complex and how many more moving parts there are in the GFCI.