Beer kit from Muntons without corn sugar

The can says to add 2.2 Lb of corn sugar to reach a batch volume of 6 US gallons and a gravity between 1.040 to 1.044.

What would be my volume if I just use the can?
What would be the gravity for that volume?

 

It is probably 1.03ish.

I would get more malt extract...

Honey or agave nectar is a better than corn sugar.

Molasses is also interesting.

If you have acid blend, you can make belgian candi sugar from table sugar...

 

At my LHBS they told me that I can use the can without corn sugar for a 4 gallon batch...Is this correct?

I understand that the gravity of corn sugar is 1.037 so if I substract 2.2 lbs of it from this instructions, what is going to happen?

 

If you go for a 4 gallon batch, yes you will compensate for the lack of corn sugar and boost your gravity back up to 1.040ish.

If you want a little more gravity and can't get to the shop, the alternatives that i mentioned work well.

 

Here's a handy conversion chart that will make it easier to determine how much grain,DME or LME replaces each. It should help a little; http://www.jaysbrewing.com/2011/11/17/lazy-chart-for-converting-dme-lme-grain/ There's also recipe calculators free online you can use to experiment with how much of what gives you the OG,etc that you desire.
https://www.brewtoad.com/recipes/new/
http://beersmith.com/
http://www.brewersfriend.com/homebrew/recipe/calculator