dgelectron:
It seems to be a conflict between libraries Keypad and GSM...
Any idea how to solve it?
It does seem to be a conflict, but it seems to be a simple name clash rather than something more fundamental.
The C++ solution would probably be to put the libraries in separate namespaces.
The easy and crude solution would be to divide the parts of your code that need to access the keypad library into separate functions implemented in a separate file that does not include the GSM library. I'd expect to need to do a bit of work to overcome the half-baked mucking about that the IDE does with your sketch before it compiles it - probably it would be sufficient to include the keypad header file in your main sketch .INO file (so the IDE recognises that it is required by your sketch) but put a #ifdef 0 / #endif around that so it didn't actually get included in the compilation (which would then provoke the error you're trying to fix).