Note on Stride:
Memory stride is the distance between memory accesses and is measured as:

|(Starting address of next memory access) - (ending address of current memory access)|

We look at two types of memory stride, local stride & global stride.

Local stride: This is the memory stride between two memory accesses for the same memory reference.

Global stride: This is the memory stride between memory accesses for consecutive memory references.

Consider the following example: 

for (i=0; i<1000; i++){
   for(j=0; j<10; j++){
       sum += arrayOne[i] + arrayTwo[j];
   }
   result[i] = sum;
}
The memory stride between consecutive memory accesses for the arrayOne memory reference is its local stride, i.e. (starting address of arrayOne[50] - ending address of arrayOne[49] ) is the local stride of arrayOne.
The memory stride between the memory accesses of arrayOne & arrayTwo is the global stride, i.e. (starting address of arrayTwo[50] - ending address of arrayOne[50]) is the global stride.

Note: This also means that if a region of code contains only 1 memory reference, global stride will be the same as local stride.

gStride3:
gStride3 or global stride 3 is a measure of the number of consecutive memory references that access the same 4 Byte memory location.

Classifying gStride3:
gStride3 can be classified as low, medium or high as follows:
BucketCondition
Low The region of code contains no memory references.
OR
The unique array references that have a global stride of 3 are outnumbered by 8:1 or more.
Medium The unique array references that have a global stride of 3 are outnumbered by 6:1.
High The unique array references that have a global stride of 3 are outnumbered by 3:1 or less.

Example:
Consider the following code: 

for (i=0; i<1000; i++){
   for(j=0; j<10; j++){
       sum += arrayOne[i] * arrayTwo[j];
   }
   result[i] = sum;
}
This code contains 3 memory references (through arrays arrayOne, arrayTwo & result) and none of these references access successive locations since they all access different arrays.
Therefore, gStride3 can be estimated to be low.