Not a silly question 
Gimp at the moment is a 100% RGB editor. The developers are working on it, some improvements to importing CMYK images, but still RGB. You will have to wait probably until Gimp 3.xx
You can (sometimes) open a CMYK image in Gimp but it is converted to RGB and you will edit in RGB and it is saved/exported in RGB unless...
No need to use an online converter, there is a converter here https://cyan.fxarena.net/ that can be used stand-alone or when run also installs as a Gimp plugin.
see: https://www.gimp-forum.net/Thread-CMYK-c...4#pid19264 (also go back to the start of that thread, it is a bit convoluted)
Since Gimp will always edit in RGB, (also true of Paint...etc) use Krita http://www.krita.org which can convert between RGB <> CMYK and edit in either mode.
Gimp at the moment is a 100% RGB editor. The developers are working on it, some improvements to importing CMYK images, but still RGB. You will have to wait probably until Gimp 3.xx
You can (sometimes) open a CMYK image in Gimp but it is converted to RGB and you will edit in RGB and it is saved/exported in RGB unless...
No need to use an online converter, there is a converter here https://cyan.fxarena.net/ that can be used stand-alone or when run also installs as a Gimp plugin.
see: https://www.gimp-forum.net/Thread-CMYK-c...4#pid19264 (also go back to the start of that thread, it is a bit convoluted)
Since Gimp will always edit in RGB, (also true of Paint...etc) use Krita http://www.krita.org which can convert between RGB <> CMYK and edit in either mode.