Martin said:"GT" stands for the integrated GPU part of the CPU (iGPU), RAPL = Running Average Power Limit
So this particular reason seems to indicate a violation of package power limit (short: PL2 or system: PL3).
More information can be found in the "Intel® 64 and IA-32 Architectures Software Developer’s Manual", which is publicly available on the Intel web. This is for advanced users.
Also note, that some CPUs might contain bugs which cause that some Power Limit Reasons might not be reported properly.