AWS defines its prices for lambda in terms of GB-seconds, and so does Azure for its prices for functions.
What does that unit refer to? Are those Gigabytes of memory allocated for a number of seconds?
The page you reference (prices for functions) explains GB-s pricing, and also some other minimums and free grants:
Resource consumption
In the Consumption plan, functions are billed based on observed resource consumption measured in gigabyte seconds (GB-s). Observed resource consumption is calculated by multiplying average memory size in gigabytes by the time in milliseconds it takes to execute the function. Memory used by a function is measured by rounding up to the nearest 128 MB, up to the maximum memory size of 1,536 MB, with execution time calculated by rounding up to the nearest 1 ms. The minimum execution time and memory for a single function execution is 100 ms and 128 mb respectively. Consumption plan pricing includes a monthly free grant of 400,000 GB-s.
So it's execution time by average memory allocation.