Thank You! It is one of my favorites, especially taken with such a simple lens.
Unfortunately I'm not an expert on GIMP (or Photoshop in that matter). I know what you are talking about because it happened to me also while I have opened the Tiff from Lightroom in GIMP (don't remember the exat message). I think GIMP (in my case at least) just couldn't read the 16bit encoded image and converted it to 8 bit. I didn't seen any noticeable downgrade in image quality though and after I've finished to edit it I just exported it as JPEG. I use 32bit Windows7, but not sure this limitation comes from that or from some other reason.
But again, I am far from understand anything on this issue and would appreciate if some one more knowledgeable could share his insights with both of us.
One more. Taken with EF-S 55-250 and edited in GIMP 2.8 and Lightroom 4.1 desaturate.

I was going to try to play a bit with GIMP, but first time I imported in, it said it was downconverting my image....can't remember the exact message, but I was shocked that GIMP seemingly couldn't handle my image at high resolution.
You're pic looks great..wondering if you found a way to bypass this downgrade GIMP seems to want to do at times...or if not.
What did you import into GIMP with as far as format, 64/32 bit....etc....
Thanks in advance,
cayenne