GVA.gget_and_update

You're seeing just the macro gget_and_update, go back to GVA module for more information.
Link to this macro

gget_and_update(table, key, function)

View Source (macro)

Get and update a variable from a table by a function. Operation is probably not atomic and won't save you from race condition. (are you sure to write a script if you have these requirements?)

function must be a function with an arity of 1.