Not everybody pays by Direct Debit. A payment made any other way is recorded here and counts exactly the same.
Otherwise you would be keeping a second list beside this one, which is not really having your subs on one screen.
Marking somebody as paid
Open their name under Members. Under Subscription, pick how they paid: bank transfer, cash, cheque or something else. Then press Mark as paid. It starts on the way they paid last time.
The Money screen does the same for many at once, below.
It writes the same record a Direct Debit writes. So the totals, the arrears list and the member's own record all agree, without anybody keeping a spreadsheet alongside.
Nothing about this touches GoCardless, and nothing here can. It is your club saying money arrived.
A pile of cheques at once
Forty cheques from the AGM, or a register that arrived already paid up, is one press rather than forty.
On the Money screen, Record money in takes you to Still owing. Open a group with See the names and tick the people whose money you have, or Tick all. Nothing is ticked for you.
Say how they paid and the date, then press Mark as paid. It shows you how many and the total before anything is written, and it records only the names you ticked.
Members who say they have paid
A member who paid by bank transfer can say so, and they land at the top of the Money screen under Waiting for you. Check your bank, then Mark as paid. If it has not arrived, Not arrived, I've chased them says so.
Waiving a sub
Sometimes the club decides somebody does not owe it. An honorary member, a hardship case, somebody who joined and then could not play.
Waive is that answer, and it is not the same as marking them paid.
The sub still happened and the record still says so, with a reversing entry against it and your reason on it. What changes is that they are no longer in arrears.
Waiving twice posts one entry, not two.
Who can see the money
Who can see the money at all is a permission of its own: the treasurer and the owner by default. See Who does what.