FCInv = Net PPE End - Net PPE Bng + Depreciation - Gain from sales
FCInv = Gross PPE End - Gross PPE Bng - Gain from sales
I was really confused over this and struggled with it a long time. Schweser does a shit job of explaining this but in one of their challenge questions in the FCF chapter helped illustrate this.
This makes sense because capex is a periodic cash out, and proceeds is a cash in, this is actually pretty much what the stmt of CF from Investing. On the other, if you are given gross or net PPE, when you sell an asset, the book value is already taken into account by the change in PPE (the BV is removed from balance sheet after sales), so you just have to account for the gain part.