The time to load individual rounds into a weapon in GURPS is indicated with an 'i'.
The format is Shots(Time to Reload), with an i to show individual rounds instead.
For example, a Henry rifle is listed as "15+1(2i)". That means 15 shots in the magazine, one in the chamber and it takes 2 turns (seconds) per shot to reload.
There are also more detailed rules in High Tech that give the times to prepare a weapon for loading and then once loaded, ready it for firing, but it tends to average out to the same times as given on the weapon table.
What does this look like in real life?