In general, to do this sort of problem, you want to find the "worst-case scenario" – in this case, the greatest number of gloves you can select that don't fit the conditions you've set out. Then, selecting any more will satisfy the conditions.
For example, selecting either white or grey socks out of a drawer, the most you can pull out without having a matching pair is two - one white and one grey. The third one must be either one of the two colours, meaning that there is at least one matching pair.
In this case, the gloves can either be left or right gloves, which puts another condition on the problem.
a) The most gloves you can have without a single matching pair is all the gloves of a single hand. So if you have more than 10 gloves (at least 11), at least one of them will be a matching pair.
b) The most gloves you can have without a matching pair of each colour is all the gloves except the gloves of one hand and one colour (so that you never have a matching pair of that colour). For example, if you have none of the left black gloves, you'll never have a black pair, and you have 15 gloves. However, it's possible to do better than this. Which colour will give you the worst-case scenario?