Is there a tool that allows for changing the time in a GPS file?
I'm certain that tool does not exist. That being said, you can open the .gpx file in TextWrangler (free) and view the time stamps and edit them. They are in the XML format:

&lt;time&gt;2013-03-21T18:04:36Z&lt;/time&gt;

I dont see why you couldn't import them into an excel spreadsheet and alter the times with an algoritm that would add a specific quantity to each. But I wouldn't know how to do that.