I’ve done a fair bit of matched betting over the years - firstly, it is not gambling - prior to placing a bet you know exactly what the outcome will be. Secondly, there is risk, but, that risk is only your own human error - don’t underestimate that (I can speak from experience here!).
As Recchan says, you basically utilise the different offers bookies put on. You basically bet both ways (it will happen, or it won’t) on a particular event, but one of those bets will be your free bet and so you make a guaranteed amount of money. It takes a fair bit of effort to manually work out how much you need to bet each way, but luckily there’s websites that’ll do it all for you. This is a great website to get you started if you do decide to do it: https://matchedbettingblog.com/
In summary, it’s not gambling, but there are risks because one mistake can be costly. I accidentally bet the same way on something twice and lost a couple of hundred pounds because of that one mistake - takes quite a while to build that back up. Also, sometimes you need to bet over £100 to make £5, so it can be quite intensive for not a high reward (will also make it damn confusing keeping track of stuff in Emma ). The first £100-200 is fairly easy to make, and then it gets more difficult to find decent offers.