Can you pass an array as input to a geometry shader?

Arrays of arrays in GLSL are read left to right. vec2[3][6] is read as "an array of 3 elements, where each element is an array of 6 elements, where each element is a vec2."

GS inputs are arrayed; each element represents a single output from the preceding shader stage. The fact that the "single output" may actually be an array type is irrelevant.

So if you're using input/output variables that are not in interface blocks, this would look like this:

//Vertex Shader
out vec2[6] someVariable;

//Geometry Shader
in vec2[][6] someVariable;

The first index is the vertex index.

Note that this may require GLSL 4.30 or the ARB_arrays_of_arrays.

If you use interface blocks to pass data, then the array goes on the instance name of the interface block itself:

//Vertex Shader
out SomeName
{
  vec2[6] someVariable;
};

//Geometry Shader
in SomeName
{
  vec2[6] someVariable;
} instanceName[];

Since the block has an instance name, you would refer to it in GLSL as instanceName[index].someVariable.

This can be done in OpenGL 3.2.