Do I have to pay for using Greenshot?
Greenshot is free in terms of the GNU General Public License (GPL). Basically, that means that end users may use this program free of charge for private as well as commercial purposes. This is probably the case for the large majority of use cases.
Restrictions apply e.g. with regard to modification, derivation and redistribution of the program’s source code. Please have a look at the licence text of the GPL; if you are not sure about its meaning, you should probably consult your lawyer, as we cannot give legal advice.