Are Gerber files of inner layers interchangeable?
The Gerber files do not specify the order of layers.  As long as you don't use blind or buried vias, the layers can be stacked in any order.  

The file names for the individual Gerber files may vary between different CAD systems, and may or may not imply the desired stack-up order.

I always included a "readme" file with my PCB order specifying the desired stack-up order.