You're not necessarily going to gain bodyfat is you don't use a cortisol blocker in PCT. However, definitely using one is recommended.
There are quite a few cortisol control products out there. The one I would recommend specifically for PCT is
EndoAmp Max - not only does it help control cortisol, but it improves the Testosterone:Cortisol ratio, boosting your testosterone (and Growth Hormone) production, which will aid in helping return your HPTA to normal as soon as possible.
EndoAmp Max is actually a part of the
Testosterone Recovery Stack, which has been designed for PCT usage, and is included in each Primordial Performance hormonal stack packages.