GMV stands for "Gross merchandise value" and represents the financial volume of sales of a given product by a company. 

For instance, in a manufacturing company like Apple, which sells different products such as iPhones, iPads and Airpods, the GMV would be the sum of the values of all the total products sold.

In a marketplace, usually the goods sold come from a manufacturer or a different company, but the GMV would be calculated also considering the sum of the values of all the total products sold by this platform. However, one should always consider that, especially in this case, GMV does not reflect this business revenue - as marketplaces would take only a small percentage of each performed sale.

The GMV represents only the total financial value summed up from all the sales performed from a company. On the other hand, Revenue (or "Gross Revenue") is the proper amount of money that will be accounted for each sale.

Let's imagine a company like Samsung but let's simply say that they sell only mobile phones - and also only one model. In this case, their GMV would be the sum of all their sales (in all their sales channels such as websites, physical stores, retailers, distributors and so on) in a given period of time. Assuming we are considering their GMV in a year, we would calculate their GMV as follows:

GMV = Sales from Channel 1 + Sales from Channel 2 + … + Sales from Channel "n"

Where 

Sales are monetary figures (i.e. USD 15,000) and

GMV is also a monetary figure (i.e. USD 500,000)

GMV = Sales from Channel 1 + Sales from Channel 2 + … + Sales from Channel "n"

Where 

Sales are monetary figures (i.e. USD 15,000) and GMV is also a monetary figure (i.e. USD 500,000)

EICOM Institute also recommends that you take a look at this article from Investopedia to learn more.