How to (not) use GPL Software
Practical Source Code Offer

Some Rules
The "complete corresponding source code" has to be made available 
It has to be made available for each and every object-code version that was distributed
If you strip down the source code offer (e.g. remove proprietary source code), try to see whether the result actually compiles
If the product is mixed free / proprietary software, consider including the proprietary parts (as object code) in the "source code package", so the full firmware image can be rebuilt without having to tear apart an existing image and ripping out those proprietary programs from there.