I doubt you have driven your car straight down a 30 degree slope! So that is not a good test.

Switchback vs straight down is not the question. Switchbacks will show up as horizontal variation with associated distance.

The question is the gometry of slope distance (triangle hypotenes) vs horizontal distance (long limb of triangle adjacent to the right angle).

If the "bread crumbs" are recorded as lat/long or equivelent, you get horizontal distance- the shorter distance.

By the way I am not a GPS user, just have done surveying in the past. A GPS seems to have the capability to triangulate and calculate slopes, but I do not think that is necassarily what you get when it tells you the distance. It is NOT what you get if the distance is shown on the screen (flat surface). Does your GPS have an option to give you both? You can probably find out if you read your users manual in detail or call the manufacturer.