Unwanted result of baked GI after enabled final gather ( v5.3.1)

You can see the weird shadows in this picture.
Usually appear on the dark side of mesh , or the face between two close meshs.
It goes worse if I put some single face mesh( fake trees with alpha transparent textures ) near by walls.
I tried the " is transparent " , " backface tolerance " , " pushoff " lightmapparameters , they can remove those weird shadows.
But they won't work if I enabled " final gather " , however , I need final gather ray tracing to improve the quality of baked GI .
Please help me .
Cheers .