There are many licenses that are free, not just the GPL:
https://gnu.org/licenses/license-list.html.
Copyleft is a strategy. It is not a requirement for a program to be free, but
strong copyleft licenses like the GPL do more to defend the freedom of
computer users by requiring that the four freedoms get passed on to other
people when they get a copy of the program. For this reason it's generally
good policy to recommend and encourage people writing programs to use the
GPL. But a person sharing a free program that's not GPLed could still elect
to voluntarily pass on those freedoms and the recipient is still just as
free. (If they didn't do that then the person would be receiving a
proprietary program.) Hence, programs that aren't copyleft can be made
proprietary but this is a matter of *defending* freedom, not if it is free.
But the whole issue of copyleft is a strategy for defending freedom - It is
not a requirement for a program to be free in the first place. Hope that
helps.
