While both Photoshop and GIMP feature "text on a curve", neither is best at it. I know many cartographers who rely on GIMP or Photoshop for most of the composition of their maps, that still choose to use a vector application like Inkscape or Illustrator to handle "text on a curve functions" as its much easier to place, alter, move, rotate and correct text and labels than any image editor. Really, everything from creating rivers and all text placement and borders is faster, easier, and simpler using any vector application. Xara Photo & Graphic Designer (my preferred vector application) allows me to place a curving line to follow any terrain, type in the text and fit it perfectly where it belongs. I can then simply copy the text, move it to another section of the map that needs a curved label, lengthen or manipulate the curve as needed, then place my cursor in the text and retype to whatever I need. Doing it this way is faster, easier, better than the same results using GIMP, IMHO.