Using GPL to Protect Your Ideas

> This means any and all changes I make while employed are automatically subject to the GNU Public License.

The GPL doesn't require a company to submit changes upstream. If you publish something as GPL your employer can still pay you to work on it but refuse permission to commit to anything other than an internal fork. Only people who have had the product distributed to them have rights under the GPL, so if you leave you have no legal recourse to get those internal commits unless you somehow cause the code to be distributed to you (which may cost a lot of money, if it's part of a piece of something expensive's firmware, for example).