There is no dedicated function. There is no “button” you have to click.
Just create a new invoice for the customer in question (Business → Customer → New Invoice) and GnuCash will automatically fill this invoice with all billable items you have collected items for (for which you already have posted [vendor] bills).
You then need to tick the “Invoiced?” column for all items you would like to charge the customer now (i. e. these items will be listed in the invoice you send out). So, you may withhold items, e. g. because they haven’t been delivered yet.
PS: Your question is warranted though. The GnuCash documentation should explain that behavior. I just struggled with that issue on my own. I’m a novice user. Unfortunately GnuCash is a lot trial and error.