Does HiDPI (Retina) technology utilize GPU resources? As I understand it should, but I can't find any explicit statement. This is important for me because I'm trying to understand which MacBook should I use - with external GPU or not. Still I can assume that the answer to the question is yes because Apple sells 15-inch with external GPU while 13-inch without it.

UPDATE:
In discussion bellow I found that my original question was wrong (also the wrong term was used - external, I meant discrete). I believed that OS resolution is an absolute setting for all applications. Now I assume that some functions work with native resolution, so it's obvious why they work slower on higher resolution.

Example of scaling to native resolution: https://developer.mozilla.org/en-US/docs/Web/API/Window/devicePixelRatio
Retina is just a marketing word for a high resolution monitor.  The trick is then that instead of just allowing the user to use a lower resolution than the display is physically capable of, macOS lies to the application about its size so it thinks that it is e.g 800x600.  All the system calls that then actually draw stuff like text and graphics then knows about the lie and can utilize the full resolution of the monitor.  

All this require GPU support (and was introduced a long way back) to work well so that is not the criteria.

The external GPU is useful however if you for any reason need more strength than the internal GPU can provide.  Games and intense visualization rendering are obvious choices.  As they consume more power they usually are only enabled when actually needed falling back on the internal GPU for normal chores.

Unless you know already that you will run these kind of programs the internal GPU will do fine.