I’ve seen some conflicting threads on using Object Normals in gdshaders, thankfully someone on the Godot Effects and Shaders discord helped out 
It was surprisingly simple, but knowledge in Discords can be easily lost/hard to find. If anyone’s searching for a solution, hopefully Google picks up this snippet.
shader_type spatial;
uniform sampler2D object_normals;
varying mat3 mvnm;
void vertex() {
mvnm = MODELVIEW_NORMAL_MATRIX;
}
void fragment() {
vec3 normal_sample = texture(object_normals, UV).rgb;
NORMAL = normalize(mvnm * normal_sample);
}