The fundamental issue
You want to know what width and height your image has to be, measured in pixels. The PPI value (or DPI, if used interchangeably) can help you determine this. It literally stands for Pixels Per Inch.
The following equation describes the relationship of the image size in pixel, in inches and the pixel-per-inch value that connects both sizes.
(Image size in Pixels) = (Image size in Inches) * (Pixels Per Inch [aka PPI])
You can solve the equation for any of these values if you have the other two, although you may have to transform it. This requires knowledge of algebra, but nothing beyond what you learn in any algebra class. Be not afraid or ashamed to admit if you have no clue about algebra, as this helps people to adjust their answers accordingly.
Adjusting image sizes in the Scale Image dialog
You do not have to use a pocket calculator to calculate image sizes, GIMP (and any other application with a similar dialog) can help. But you have to know about the principles, that is what value you want to calculate, and why.
Changing resolution, size in pixels constant
For example, assume someone gave you an image, which is not supposed to be changed at all, and tells you "set the PPI value of this image to 300 PPI!". You want to do this, and tell this person what the results are. So you do the following:
- Set the Image Size unit selector to something other than "px"
(pixel), e.g. "in" (inches).
- The values in the Width and Height entries will now indicate the
image size in the unit chosen (e.g. mm, and right below this area
you'll see the image size in pixels.
- Now, change the X and Y resolution values to 300. You will notice:
- The inch values in the size entries change, but the size in pixels
remains the same
Then you can tell the person who told you to modify the image: "Task done, the image would be of inches wide and of inches high if we printed it at 300 PPI, for example".
This is expected. Your are telling GIMP to change the image resolution, and this is not supposed to alter your image besides updating the PPI value.
Many image formats are able to store this value (or values, as they can be different for width (X) and height (Y) of an image), and some other applications used them to display images at this intended height.
Now, for the next round:
Changing size in pixels, resolution remains constant
So now we assume a person gave you an image, and told you "this image is supposed to be printed at 300 PPI to be A inches wide and B inches tall, can you adjust it accordingly?".
- In the Scale dialog, take note of the current image size in pixels.
- Make sure the Image Size unit is set to something other than "px",
e.g. "in".
- Make sure the resolution is set to the specified PPI value, "300" in
this example.
- Now change the inch value of the Width and Height entries, to
whatever values A and B are, respectively.
- See how the image size in pixels, as shown right below those
entries, changes accordingly.
This is expected as well, as you are now telling GIMP that you want the image to be a specific size in inches at a specific PPI value. And you are in the scale image dialog, after all.
You can tell this person: "Ok, this is doable, but the image will now be C pixels wide and D pixels tall - this is a significant change from its original size in pixels, and any scaling will affect the quality. If you had an image that was C pixels wide and D pixels tall as an original, we could use this instead."
As you can see, you may use this to determine what size your image has to be in pixels in order to achieve your task without scaling, provided that you know the expected image size in inches (or millimeters, meters or light-years).
As your image comes from Blender, you may be able to generate it at any size needed, and this would indeed be the best approach.
Changing the PPI
If you want to achieve the opposite, i.e. having the PPI value change according to the size you specify in inches, and the size in pixels to remain the same, you can choose the Print Size dialog to do this.
Addendum
As you have correctly realized, providing just one of the relevant values, the infamous "please send the images at 300 ppi (or dpi)!!!1!" lacks essential information. If this case, it has to be assumed that the person sending the image ought to know the intended image size, for example "[A4] at 300 PPI", or that the person requesting the image has been taught this is sufficient and never questioned why.
TL;DR
Connecting pixels and inches really comes down to one single equation:
(Image size in Pixels) = (Image size in Inches) * (Pixels Per Inch [aka PPI])
People with some knowledge in maths usually can't understand why someone doesn't get this equation or can't transform it, e.g. to get the size in inches if they got size in pixels and the PPI.