preload is a GDScript feature, which basically calls
load behind the scenes when the game starts. You won't find the exact equivalent in GDNative, which is C/C++. However, you can
load resources by including
<godot_cpp/ResourceLoader.hpp> and calling
ResourceLoader::load("resource path"), which returns a
Resource that you can cast to the resource type you need (see
Object method for checking if the type is correct before casting).
On a personal note, while I understand it's for having an example of the bunnymark project in C++, I would advise against hardcoding resource paths in a GDNative library (and prefer GDScript or properties for this data/high-level stuff), unless you really want to only use C++ as your scripting language :)