Matched betting definitely is gambling! It is just gambling against other people and not a bookie.
The difference between matched and normal betting is that the middleman is taken out and, for every loser, someone else wins. However there is generally a (smallish) administration fee.
For example, if I think that Manchester United has a 3/4 chance of beating Southampton, a 3/16 chance of drawing and a 1/16 chance of losing, I can offer you 1/4 (say) on Manu, 12/1 on Southampton winning and 4/1 on a draw. All of these are below my theoretical probabilities and if people take all of them (in the right ratio) I am guaranteed a profit.
Effectively I have become a mini Ladbrokes.
However, it takes immense discipline to make a little money, and it is very easy (and sometimes inevitable) to often end up taking a view (ie gambling).
Unless you love that kind of thing and have a facility with ratios, I think you would have to work immensely hard to make not much money.