Grid Fill creates slanted edge loops

Hi Jonathan. Good question!

Grid Fill has some settings, for instance you can change the Span:

I know it says number of columns here and it’s actually the number of rows, but when you change the offset, it will be ‘correct’.

As you might remember, all the Vertices of a mesh are numbered and depending on how something is created two identical looking meshes can have different numberings and then the Grid Fill will fill differently:

or:

This is why it can behave inpredictable, but just tweaak the settings untill you get what you like; it certainly beats filling everything in by hand;)

(I have the Grid Fill in my quick favorites menu because I use it quite a lot and there is no shortcut for it…)

“Grid Fill” has two basic working modes: 

 The first one is the one described above by @spikeyxxx where you select all your vertices around the hole which you want to close and where you can tweak the filling with the settings for “Span” and “Offset” after the fill command.

The second one is where you select on your hole rim two parts with an equal number of vertices. The two unselected parts should also have an equal number of vertices:

When you now select  “Grid Fill”, Blender connects the marked rim parts as it would do with “Bridge Edge Loops” and does the same with the unselected rim parts:

The “Span” and “Offset” settings now have no effect since you’ve already given Blender a hint on how to fill your gap. Like this, you obtain an absolutely predictable result.

@spikeyxxx, present this in a gallery with some meaningfull description and demand a pile of money for it 😉😁. 

But seriously: You certainly want to choose two rim parts that are orientated rather like two opposite edges of a planar or curved rectangle:

And with a distorted hole:

a) Without “Simple Blending” (more smoothened and curved connections):

b) With “Simple Blending” (more straight connections):

@duerer I understood that, but why aren’t they (the unselected parts) bridged with straight lines?

Not important though, who would want to use Grid Fill like that;)

According to the Blender 2.83 manual, in the case that you select the whole rim around the hole, the active vertex defines per default where the corner of the filled-in grid is located and the “Offset” allows you to rotate that corner.

@blanchsb I believe this was actually the only way to use Grid Fill when it came out and it can handle quite extreme situations like J. Williamson demonstrates here: https://www.youtube.com/watch?v=pJCDmPxbwgs

I completely forgot about this and  use it almost exclusively  to fill (mostly planar) holes. I have been missing out!

I remember having seen this video some time ago. Excellent demonstration and comparison with alternative methods!