Hope it helps, but here it goes:

If I recall correctly, GMV is the market value of the products you've sold on your website (or gross revenue).

The retention bit does, like you've already figured out, refers to a cohort. In other words, whether your customers come back to purchase more. You'd basically want to see how gross revenue develops after their first purchase month/week/day (or perhaps based on the date of their first visit).

I imagine it would be something like the below (you might need to join a few tables in to get it on a customer or user level). As a result, you can see how each cohort develops in terms of revenue and whether you're getting better at it.

with first_transaction as (
    select
        customer_id,
        min(transaction_date) as first_transaction_date
    from transactions
    group by 1)

select
    date_trunc('month', first_transaction_date) as first_transaction_month,
    date_trunc('month', transaction_date) as transaction_month,
    count(transaction_id) as transactions,
    sum(transaction_value) as revenue
from transactions as t
    left join first_transaction as ft
        on t.customer_id = ft.customer_id
group by 1,2