The wiki page https://wiki.gentoo.org/wiki/Rtl-sdr seems outdated: a "sdr" group is not created by emerging net-wireless/rtl-sdr, the "usb" group replaces it. More importantly, the package net-wireless/cubicsdr would provide a graphical interface, but there is no such package in portage. So, the question is: is there some package providing similar functionality?
Thanks Jorge Almeida
