Your gloss map is basically your roughness map.
You plug it into OneMinus and then plug that into Roughness
The reason you are plugging it into Oneminus is to invert it.
In UE4 I believe, 1 means black and 0 means White. That is how UE4 handles materials.
1 = A surface is rough.
0 = A surface is not rough.
Here are some links of reference that could benefit you. I urge you to plow over:
PBR Practice
Tutorials & Resources | Marmoset (Almost everything you need to know is here)
PBR Theory
PHYSICALLY BASED SHADING IN UE4
Physically Based Materials in UE4
DONTNOD Physically based rendering chart for Unreal Engine 4
RYSE – The Transition to Physically Based Shading