Karlwunsch, try Raw Therapee – it’s a way good RAW Editor for Linux you’d get – and OpenSource, so free, too. GUI is close to Lightroom, but without that catalogue feature.
http://rawtherapee.com
Another great one is Dark Table, DT in short – which is only avialable for Linux, not for Windows, but also for OSX. RT in fact is avialable for Linux, OSX and Windows.
Dark Table 1.6 is current, and RT 4.2.1 at stable status…it’s quite good, if not better than RT….well, it depends what you’d personally prefer.
http://www.darktable.org
The only thing on Linux i’m waiting for aeons is 16bit TIF support with GIMP.
Started out doing photography at the age of 6 using an uncle's old 1940 kodak brownie box camera. At 15 years of age, I decided to buy my very own 1975 Praktica SLR camera. I now shoot with a Nikon D850. I do unpaid TFP and commercial paid work.