Depends on your use cases and the ventilation of your case. GPUs are generally okay with higher temps than your CPU. If the GPU gets enough airflow it's okay though not really sensible when doing a custom loop. You already spend quite a chunk of money for radiator, pump, reservoir, tubing, fittings and the CPU block - just spend another 100 bucks for the GPU block and a few bucks on 2 more fittings and be done with it.
Is it efficient? Depends on what you're planning and what your goals are. From a monetary point of view it is never efficient. But noise levels and temps will improve with a reasonably designed loop.
AiOs will at best lower your noise levels (after exchanging the included fans) - they are usually not better in regards to cooling performance compared to the best air coolers available which still cost only half of a semi decent AiO if not less. AiOs are usually never a good option for GPUs. Once you replace the included fans with better ones you're reaching price levels that could give you a small custom loop (look at the EKWB Fluid Gaming kits - beware though, they are made of Aluminium and do not mix Copper - they are now EoL so buy now on the cheap or ignore it).