I have only some hints of what you can check but I can';t give a complete answer.
If your keys are automatically detected then you will see a similar output
$ xmodmap -pke | grep XF86Audio
keycode 121 = XF86AudioMute NoSymbol XF86AudioMute
keycode 122 = XF86AudioLowerVolume NoSymbol XF86AudioLowerVolume
keycode 123 = XF86AudioRaiseVolume NoSymbol XF86AudioRaiseVolume
keycode 171 = XF86AudioNext NoSymbol XF86AudioNext
keycode 172 = XF86AudioPlay XF86AudioPause XF86AudioPlay XF86AudioPause
keycode 173 = XF86AudioPrev NoSymbol XF86AudioPrev
...
This means that your keys are recognized and mapped by the kernel and X. If this happens then you can tie these XF86 keyboard symbols (for example XF86AudioPlay) with specific commands.
For gmusicbrowser the commands to do things are of the form
$ gmusicbrowser -cmd NextSongInPlaylist
See full list of commands
$ gmusicbrowser -listcmd
I think, the way to tie a XF86 keyboard symbol with a command is desktop related (for example in xfce http://docs.xfce.org/xfce/xfce4-settings/keyboard - Application Shortcuts) I don't know if there is a generic way to do this.
You can check https://wiki.archlinux.org/index.php/Extra_Keyboard_Keys to see more details about the procedure of identifying keyboard keys.
If your keys are automatically detected then you will see a similar output
$ xmodmap -pke | grep XF86Audio
keycode 121 = XF86AudioMute NoSymbol XF86AudioMute
keycode 122 = XF86AudioLowerVolume NoSymbol XF86AudioLowerVolume
keycode 123 = XF86AudioRaiseVolume NoSymbol XF86AudioRaiseVolume
keycode 171 = XF86AudioNext NoSymbol XF86AudioNext
keycode 172 = XF86AudioPlay XF86AudioPause XF86AudioPlay XF86AudioPause
keycode 173 = XF86AudioPrev NoSymbol XF86AudioPrev
...
This means that your keys are recognized and mapped by the kernel and X. If this happens then you can tie these XF86 keyboard symbols (for example XF86AudioPlay) with specific commands.
For gmusicbrowser the commands to do things are of the form
$ gmusicbrowser -cmd NextSongInPlaylist
See full list of commands
$ gmusicbrowser -listcmd
I think, the way to tie a XF86 keyboard symbol with a command is desktop related (for example in xfce http://docs.xfce.org/xfce/xfce4-settings/keyboard - Application Shortcuts) I don't know if there is a generic way to do this.
You can check https://wiki.archlinux.org/index.php/Extra_Keyboard_Keys to see more details about the procedure of identifying keyboard keys.