Sell.
The main thing you need to watch when cutting is your caloric intake. The gainer is gonna bust this wide open.

If you are in for the protein, get a protein tub instead. Fitting in a mass gainer is counterproductive.

400 calories in the end is 400 calories regardless of the source. But like the poster above said, nothing beats natural food. And your sentence near the end contradicts yourself; you mentioned you are cutting and need to be in caloric deficit, yet you mentioned you want to increase weight?