Muay Thai gloves are measured in ounces. 1 ounce is 28.3495 grams. All gloves will come with a weight specification, and it's usually marked on the palm side of the cuff. The weight corresponds to the whole weight of the glove - cuff, padding, leather etc..
Expect variance when buying handmade gloves, as no two pairs are the same. For example, you may buy a 16oz pair of gloves and one of which may weigh 16oz exactly, while the other weighs 16.5 oz. This isn't an issue when overweight, but it can be if they are underweight (i.e. state they are 16oz but actually weigh 12oz). This is only really an issue with poor quality gloves. All the brands we stock work diligently to ensure accurate weights and measures. You can easily measure them on a scale.