It's harder than measuring CPU load, and depends a lot on context. For example, often 90% of a GPU's available flops are exclusively for low-precision matrix multiply-add operations. If you're doing full precision multiply-add operations at full speed, do you count that as 10% or 100% load? If you're doing lots of small operations and your warps are only 50% full, do you count that as 50% or 100% load? Unfortunately, there isn't really a shortcut to understanding how a GPU works and knowing how you're using it.