At the start of a match you will be given the option to bid using gems to secure your right to have first kick-off.
What is important to note is that this is simply a 'bid' to have first kick-off and nothing more. It is not a 'bet' to see who will win the match, so if you are lucky enough to bid the highest amount and secure kick-off, you will not see a return of your bid or any additional amounts for winning the match.
Consider it more like an auction where people outbid each other, but they don't get any money back as a result.
If both users bid the same, then the kick-off winner will be chosen at random.