This is the thing, gaining mass while cutting weight is almost impossible, this is the reasonw why, when you bulk you eat more calories in order to get stronger, pushing more weight, and using the extra calories to help you recover faster. Going this way you tend to gain a little bit of extra fat because your intaking more calories than your burning throughout the day. When you cut you try and get your calories burned to be greater than how much you take in without damaging your muscle mass (i.e more protein and lower carbs in order to make sure the muscle have enough protein to sustain themselves while your taking away some carbs, which are used more readily as energy than protein).
However, this isn't to say that it is completely impossible to increase lean mass while dropping fat, but what you SHOULD NOT do is take a gainer trying to do this because of the higher fat content, sugars and carbs. What you should do is increase your protein intake and slightly decrease your carb intake. Remember protein is the building block of muscles, not carbs, but carbs do play a role in absorbing protein into the muscle, but not that many carbs are required. So aim to get 55% of your calories from protein, 35% from carbs and 10% from fats, this will help you drop the fat (assuming you are also doing your cardio you should be doing to drop fat, if your not, better start) and doing cardio after your workout will also allow your body to deliver more oxygen into the muscles you just worked and also deliver more nutrients once you take your post workout shake.
Instead of the weight gainer, use
100% Whey (most people including myself prefer the ON brand, so what I am recommending are all ON products unless specified),
Casein protein, BCAA's, MultiVitamin, creatine and glutamine. Aim to eat every 2 hours, this will allow your body to metabolize faster and burn more calories but give your muscles enough protein in order to grow and sustain. The creatine and glutamine will allow your body to help sustain strength and prevent muscle breakdown, and yes you can take creatine and cut weight successfully. So by increasing your protein intake you can increase your lean muscle mass, but don't look to put on a lot of muscle while cutting fat, because the diets to do so are different. So chose which one you want to do and go that way.
Enjoy!