I have had GIMP in the past, but a lot of what I tried seemed counter intuitive! Can GIMP work non destructively on raws? I seem to recall the need for thingy's raw plugin which I also recall I couldn't get to work.
If you use GIMP, be aware 2.8 it's still a bit too tied to the sRGB colour space. It may not matter much if you output into that color space and/or doesn't edit much colors, but if you do and/or print on many pro photo printers it may become an issue, depending on the images.
Also AFAIK 2.8 doesn't support 16 bit image editing, which again may be an issue for your images or not - depending on how you convert RAWs for input.
AFAIK GIMP can use Darktable and UFRAW as RAW converters - but some of them don't work on Windows, and may need other pieces to work.
Version 2.9 - which is not a "stable" one yet - improves color management and adds 16/32 bit file processing.
Adjustment layers I'm afraid are planned for a future version (see https://wiki.gimp.org/wiki/Roadmap
), and GIMP development is not really fast... my take on GIMP is "if it does what you need, use it, but don't expect missing features to be added quickly".