The GFCI will attenuate the signal yes however it is something you can still attempt since most adapters (including the TP-Link) do have specified speeds they will likely use when passing through a GFCI outlet that are far greater than what is needed with OpenSprinkler.
Some of the data I see suggests speeds of 14-18 Mbit when using GFCI some suggests higher. I think it is worth trying and if it works, great otherwise find another outlet.
I hope this helps!