oh, well that explains. I did not know not implementing a function in .cpp too would lead to “unresolved external symbol”, I understood it as “there is no beginplay() to override”, and never thought adding the .cpp side of things would fix it.
I guess I learned something much more general and fundamental than I thought I would 
Thank you so much! 
EDIT: I successfully made the gamemode print “Hello World!” on BeginPlay(), and you can’t tell how happy it makes me :P. I finally “did a thing” in C++ 