I've heard up to 80%. However, a quick search through SourceForge (the most popular repository for OSS, where most of the software is GPL) will show you that a vast majority of those projects are nothing, most not even out of the beta stage. There are about four times as many projects that have not progressed to production/stable/mature as have.
Let's look at software that matters: Linux, gcc, Apache web server, Firefox, Thunderbird, Tomcat, Python, Perl, SpamAssassin, MySQL, PostgresSQL, PHP, FreeBSD, CVS, X-Windows, Solaris, Darwin (OS X core). I can't think of any other open source software off the top of my head right now, can you?
Four of those are under the GPL, and MySQL has an alternate commercial license. The rest are under other licenses.
Openoffice.org uses the LGPL.
Mono, Wine, Yast, Gimp, Hurd, and how could you forget Open Office? It's GPL isn't it?
Most are Gpl because it has a sneaky little clause the Free Software Foundation lawyers use to confiscate code from others.