You can get portable plug-in GFCIs in a variety of forms. There is the
device that you plug into a wall, and which has an outlet that you plug
your load into. Here's an example:
There are also versions built into the male plug of an extension cord,
or ones in the form of a box in the middle of a short extension cord. I
have used several of them in these various forms.
All of the portable ones I've used behave differently from wired-in
GFCIs in one respect: they disconnect automatically when input power
goes away. When you first plug them in, you have to press Reset to
activate the outlet. And if power goes away momentarily during use,
they drop out and stay out until manually reset, even if power comes
GFCI breakers don't do that, nor do GFCI outlets designed
for permanent installation in a wall box. (Some recent wall GFCIs I've
installed require power to be present on the inputs to reset them, but
once reset the internal switch remains closed when power fails).
This can be put to good use with any line-powered tool that you'd rather
not have restart on its own if power goes out unexpectedly. For
example, if you're using a table saw and the power fails, you'd probably
rather have the blade stop and stay stopped, rather than restarting at
some unforseen and unpredictable time a few seconds later.
Industrial machinery often has this feature built in as a side effect of
using a "magnetic starter" to start and stop the motor. The starter is
basically a high-current relay with a set of pilot contacts that hold
the relay latched on until a Stop button or safety interlock is opened.
The hold-in current comes from the line, and if the line voltage goes
away the relay drops out and stays off.
A portable GFCI is a relatively inexpensive way to get the same feature
for 120 V 15 A (or less) machinery. (Of course, you don't get any of
the other features of a magnetic starter - just this one).