This edge loop (the roof of the car I'm trying to create) consists of 34 vertices on each long side, and 8 vertices on each short side. Together with the 4 corner vertices that makes 88 vertices in total. I'd say the perfect set-up for a Grid Fill ... except that it does not work (see error at top of screen). I checked for double vertices, there are none. I checked if the loop is water tight, by using ctrl+RMB select, and that's all ok. So what could be the problem?
As stated by the Blender manual's section about the Grid fill command:
Grid Fill uses a pair of connected edge-loops to fill in a grid that follows the surrounding geometry.
Despite this, using the vertices indices and the picking order, the operator is capable to guess the user intention even if he defines the whole boundary, but it still needs to guess the right offset's and span's values because you are not providing edgeloops but only some vertices and it has to guess which gruops of them belong to an edgeloop.
See in this sequence how the operator behaves differently on same geometry simply by picking another vertex to complete the selection.