Let's start with some definitions.
For most people in the software field, "open source" refers to software released under a license that is defined as open source by the Open Source Initiative, and (to a lesser extent) "free software" generally refers to software released under a license that the Free Software Foundation considers free. These aren't the intuitive meanings, but you'll just confuse people if you use them with other meanings. Windows is not either free or open source by these definitions.
If you disregard the license, that's not going to help you legally. To quote the Gnu General Public License, version 3, section 9,
You are not required to accept this License in order to receive or run
a copy of the Program. Ancillary propagation of a covered work
occurring solely as a consequence of using peer-to-peer transmission
to receive a copy likewise does not require acceptance. However,
nothing other than this License grants you permission to propagate or
modify any covered work. These actions infringe copyright if you do
not accept this License. Therefore, by modifying or propagating a
covered work, you indicate your acceptance of this License to do so.
Section 2 supplies the basic permissions, including
You may make, run and propagate covered works that you do not convey, without conditions so long as your license otherwise remains in force.
Conveying under any other circumstances is permitted solely under the conditions stated below. Sublicensing is not allowed; section 10 makes it unnecessary.
I've removed the material that gives you slightly more freedom when subcontractiong development. Clearly, you can change the software without any further permission, and you have permission to redistribute under the terms stated later in the license.
You can read Wikipedia on US copyright law. There are a large number of things you aren't allowed to do, and an agreement that allows you to do those things anyway is generally called a license.
Therefore, if you redistribute a GPL-covered work without complying with the license, you don't have a valid license, and you're committing copyright infringement. The section "Infringement" should summarize what you might face if you violate copyright, which is injunctions and possible statutory damages.
Other problems are possible. The Digital Millennium Copyright Act has provisions to allow a copyright holder to ask a web host to remove content from their site. Depending on what you're doing, you might face criminal prosecution, although I wouldn't expect that to come in unless you're particularly flagrant.