Access to /dev/ttyUSB0 and sudo
Solution 1:
As suggested by CheddieMerai, using ls -l gadumaster may tell you that you set incorrect file permissions. This is not an issue with the USB connection anymore.
You can resolve this by rebuilding your application or by changing the file permissions via chmod 775 gadumaster (or similar) and/or chown $USER gadumaster (you can replace $USER with the user supposed to "own" the file).