Recording transactions that happened before the opening balance

[Jacques FLORENCE]

Hi,
I just started using gnucash.

TLDR: how to track transactions that happened before the opening balance,
without modifying today's balance?

So let's say I did setup an account with opening balance $1000 at the
12/07/2016.
Let's say that's the amount I actually have in my bank account right now.

I will record all this account's transactions from now on in the future.
However, although I don't care about transactions prior to today, there are
a few of them that I would like to keep track of.

Let's say I received a $200 gift from my mom three months ago, and I would
like to register it. So I add a $200 transaction in the account with date
9/07/2016.
(Let's say that it's important that I know the total amounts of gifts
received from my mom during my whole life, as indicated by the total amount
of income:gifts:mom'sGifts...)

The problem is that now my account has a value of $1200. But this is wrong!
My account has a balance of $1000 right now as indicated by my bank
statement.

One solution would be to modify the initial opening balance transaction to
$800. But my balance right now is not $800, so I should change the date of
this initial opening balance. But what should this new date be? Certainly
not 9/07/2016: my account balance at that time did not have that value, as
I omitted many transactions inbetween.
Another problem with this is that if I want to add some other transactions
of the past, I will have to readjust this opening balance transaction every
time...

Is there a way to record transactions that happened before opening balance,
without changing today's balance

Thank you for your help

Jacques