I know that dfgi is not finished yet but it is good enough for my project .
My question is : Am I allowed to use it in my commercial project?
sorry if this is in the wrong section
You are allowed to use every of the UE4 in your commercial project 
Here you can see how DFGI works -> https://forums.unrealengine/showthread.php?69307-The-state-of-Distance-Field-GI-in-4-8
Yes you can, anything available in the engine can be used as governed by the EULA license, any/all features in the engine can be used.
It’s not so much a matter of can you, it’s more should you (as you mentioned, it is not fully supported). That part is up to you and your projects needs, but I will say DFGI seems to be far enough along development for it a viable option.
EDIT: looks like out ninja’d me…
Thank you and 