So, it begs the question. If the gnomes (or toy trucks, or tea kettles, etc) are just things left behind by random people on public land, who would get in trouble for picking them all up and hauling them to the nearest dumpster? You would certainly be leaving the area better than you found it.
I would not recommend this course of action, particularly if you want to be part of a respected community.
I don't understand why people have put those things there, but they represent a tradition to a number of people, and those people would certainly take offense. I think their offense at losing their tradition would be substantially greater than anyone else's offense at the tradition.
I have come across comparable things, though on a smaller scale, that are clearly grave markers or places where the ashes of the deceased were probably scattered. The things such as toy cars and small plaques certainly are not native to the location, but to take them away to satisfy my own desire for nature and nothing but nature would be self-centered and thoughtless.
A far better course of action would be to learn the history of the gnomes, or tea kettles or whatever. Then perhaps one could at least understand what others are doing and why. It may not be worth embracing, but it could help to differentiate the trash from the curious.
I don't have a problem distinguishing trash. It is the stuff that was thoughtlessly (at best) left behind. The gnomes and tea kettles required forethought and deliberate planning in order to arrive there. Someone (or a bunch of someones) had a purpose. Far be it from me to play "holier than thou" with their purpose. Anyone who does is likely to bring a backlash down on their own backs and quite possibly on the backs of all those with whom they are associated.
Don't mess with what you don't understand, and don't adopt a course of action which you cannot fully reverse.
My two cents, and that as someone who has spent hundreds of hours on Adopt-A-Trail cleaning projects.