That's great. A good trainer will motivate you and push you to follow through with your workouts 100% of the time instead of just 90%. You'll see how that make a big difference.
Your trainer will also make sure you are pushing yourself hard enough because they'll have a better idea of what you are truly capable of in the gym.
Hitting the targeted muscle group directly, effectively, and efficiently with every rep, set, and exercise is extremely important--and your trainer will help you do that too.
It can't be rushed so, the clean weight you gain is the clean weight you gain.
The standard foods like chicken breasts, tuna, rice, vegetables, and good fats like flax seed oil are some of the foundation foods of any good diet.
You are going to have to "do your homework" and figure you your basic meal plan.
From my experience, the very best people in bodybuilding and fitness generally stick to the same effective foods that fit their caloric and macronutrient goals and requirements--and then add extras occasionally from there.
I assume the "25" means 25 grams of protein that you can figure out from the food package or a conversion chart on the internet. The person who did your diet for you is estimating that there are 25 grams of protein in that particular food source in every 4 ounces.
The good thing is that, once you've spent a couple of hours figuring this all out initially, you'll have that knowledge forever. You don't have to do it over and over again every single day of your journey.
Train Hard. Think Big.