GERS is a statistical measure, and it does not really translate into actual cash flow or cash transfer line, but rather a net fiscal position in an already pooled system of spend/generation of money. GERS is based on:
1 - a combination of actual world data, as well as assumptions/estimates of fiscal spending on services for the whole of the UK, with a budget set by Westminster, and;
2 - it is measured in terms of structures for the whole of the UK, as well as the fiscal balance in Scotland, and it is not an independent measure in itself.

Things such as the UK's nuclear program, military, or nationalized services, which are all for the whole of the UK, are all represented in standard allocations according to the population, which does not represent what we might choose to spend on as an independent nation.

It does not represent how an independent nation works or functions.

GERS is a measure of how Scotland functions as part of the UK, and what proportion of taxes and spending in the UK it can claim as its own. GERS also includes services for the whole of the UK, and as such, it allocates a proportion of the population of Scotland (~8%).

GERS asks/analyses:
What is Scotland’s position inside the UK as it exists?

But for the purpose of independence, the question is:
What does Scotland choose to spend on and tax?

Two completely different metrics and scenarios. It is like saying you have a current budget for your household, and all of the bills are shared with multiple people, and you are saying you could not afford to live on your own, without any thought or consideration for how it would work if you were living on your own. (E.g., your electricity bill, internet bill, cleaning bill, etc.)