Nothing can be as power efficient as an ASIC, which is designed for a specific purpose, instead of being a programmable device intended to be suitable for a large class of applications.
A GPU is much more efficient than an FPGA for what a GPU does. On the other hand for applications for which the set of primitive operations implemented in hardware by a GPU is not a good fit, an FPGA can be much more power efficient than a GPU.
For applications that involve a massive amount of computations with FP32, FP16 or BF16 numbers, for which GPUs have special hardware execution units, i.e. for training and for inference with non-quantized models, there is no chance for an FPGA to be more efficient.
If the GPU is recent enough to have good support for more heavily quantized data types, e.g. INT8, FP8, NVFP4 etc. an FPGA also does not have chances to be competitive.
An FPGA could be more efficient than a GPU if either it is some special AI-oriented FPGA, which instead of having traditional arithmetic units oriented for DSP applications, has execution units implementing the quantized data types popular in ML/AI, or if it implements inference using some new not yet standardized data type, for which GPUs do not have dedicated support yet.