- Who is responsible for supplying source code for an application,
which has been developed for a client using GPL code? I know I would need to release the binary to the client with source, but what about request from others like the original author of the GPL code. Would I forward these request to the client or would I have to support any request myself?
If you release source code to your recipients, you have no other duties, according to point 3.a . Nodoby can legally ask anything.
- How would the scenario above be affected by an NDA?
It depends on what the NDA is. Who signed it, and what subject matter cannot be disclosed according to the agreement.
If the client releases a binary only of the application where does this leave me.
If your client is not the sole copyright holder on the package, he must follow license terms of the software it's not the copyright holder of. If it received it under GPL, the GPL must be obeyed. Otherwise, your client is in violation of copyright law. (client is assumed to be a company, hence "it").
/alessandro