I've seen several statements that the zlib license is compatible with the GPL. But this does not seem to be correct. The zlib license contains the following clause:
"Altered source versions must be plainly marked as such, and must not be misrepresented as being the original software."
For a license to be GPL compatible, it must not contain any restrictions on modifications that are not also found in the GPL. But the GPL has no restriction on modification that is analogous to the first part of this clause.
The GPL contains the following clause (5a in 3.0):
"a) The work must carry prominent notices stating that you modified it, and giving a relevant date."
But this is a restriction on distribution. You could comply with this term by never distributing your modified code or by adding such notices prior to distribution. But this would violate the zlib license.
So how is the zlib license GPL compatible when it contains a restriction on modification that is not like any restriction found in the GPL and the presence of additional restrictions is what makes a license not GPL compatible?