In remote sensing, ground sample distance (GSD) refers to the dimensions of a single pixel in an image as measured on the ground. The [calculation][1] of GSD uses the focal length (FL in figure below) and sensor array dimensions, which are properties of the sensor, as well as the distance between the sensor and the ground *at the moment of image capture* (D in figure below), normally represented by the altitude of an aircraft or orbital distance of a satellite. 

Here is a visual aid to illustrate my examples: 
[![enter image description here][2]][2]

So when the ground is flat, things are as you described and the distance D is similar across the entire image which means that spatial resolution and GSD are equal across the scene (("1" above). However, since spatial resolution can be used to describe the dimensions of a pixel relative to any object in focus, consider two other scenarios. First, some objects can stand relatively high above the ground compared to the altitude of a flying craft (i.e., large buildings or trees) ("2" above). Second, image footprints can  cover large areas which contain variable surface relief ("3" above). In both of these scenarios, the spatial resolution of objects across the image would deviate from the correctly calculated GSD in such a way that objects closer to the sensor than the ground are more resolute, and those farther away are less resolute.

So in short, GSD is a standardized way to describe the resolution of remotely sensed aerial imagery, and is a metric of spatial resolution, while [resolution][3] without georeferencing typically refers to solely to the number of pixels in the sensor array, which is the product of the height and width and usually expressed in megapixels (i.e., 8192 × 5460 = ~45MP). The former describes the size of the smallest detectable object *on the ground* within a particular image, while the latter describes the level of detail recorded by a particular sensor.


  [1]: https://support.pix4d.com/hc/en-us/articles/202559809-Ground-sampling-distance-GSD-in-photogrammetry
  [2]: https://i.sstatic.net/5fwjO.png
  [3]: https://en.wikipedia.org/wiki/Image_resolution