GMaps2GPX is a tool that makes it very easy to extract the route and calculate the directions for a trip in Google Maps. GMaps2GPX will extract the directions from the address and then export them as a GPX file. 

You can convert your maps to GPX files for use in a GPS receiver. It works as follows:

Once there, click the Export button

Select your desired option (Map, Driving, Cycling, etc.)

Now, enter the address or location you plan to go to.

You can find more info about the process in the settings section of the app.