I thought the big problem is that gimp uses only rgb color encoding, which is fine for non-print, but if you want to print something professionally you need at least cmyk.
RGB color in Gimp is fine for me because I only use it for photo processing, and cameras shoot in RGB so you aren't losing any information by doing your photo processing in 16-bit or higher RGB.
You can do color space conversions to CMYK after that stage.
On an interesting side note, realistically though, I've found the vast majority of people I've had to work with don't understand the difference between RGB and CMYK and just want "PDFs" and don't necessarily let me choose or interact with the printing agency directly, or the printer is some friend's wife's father's friend's friends' friend's company on WeChat that is going to be doing the printing at 1/10 the cost of every other commercial printing agency out there and they've chosen to use that company and it would look silly of me to suggest to use a company that costs 10X more just so I can get proofs. In those cases, I've found that if I don't have access to proofs, these days, RGB PDFs often seem to get more consistently rendered than CMYK PDFs.
Yeah if you need to work in CMYK you're kind of stuck. Personally, if I needed to do a lot of print work then I'd pick up an old Mac and an old license for Photoshop CS6.
