CtrlF > Grid Fill works like this:
- It calls the (white) active vertex when it is invoked a "corner". (Red zones)
- It takes the next
Span - 1 vertices, and bridges them to their opposite numbers. (Lilac)
- It skips another corner, and then connects what's left cross-ways. (Green).
This results in
Span face-loops across the piece, (up, in the picture :) ) and 4 corners.
The illustration shows
1 on a 16-circle, with the active vertex in white.
Offset setting steps the starting-point around the perimeter.)
So you can see, to get a tidy result from Grid Fill, you need:
- An even number of vertices
- A reasonably nice correspondence of vertices to their opposite numbers, once you've decided where the first corner should go.
In your case, you need to fix the second condition.