Gerber Viewer With Good Measurement / Distance Tools?
I Like Viewmate. I always use it as the last check before my Gerbers go to the factory. Often I still find some detail which I had no seen and fix the PCB.

Load gerbers with File->Import->Gerber...
Then I select all the gerbers as a group.

I just found that 'd' starts the measurement tool (Despite the menu showing a capital-D)

Example below: