The impact is negligible with how payment gateways use different exchanges to pull out fiat, which directly results in a situation where their sales are distributed amongst several orderbooks and for that reason don't directly impact the market.
If you also take into consideration that most of the payment gateways actually prefer to hold their coins, then you'll realize that there is little to no selling pressure going on. What they do is short a small percentge of their sales to preserve their fiat value in case the price goes down. On top of that, BitPay needs coins as well, so they won't ever sell everything they have because of how certain merchants prefer partly BTC settlements instead of fiat in full.
Another thing is that if they do end up selling to take out fiat, they won't blindly dump their coins at once but will patiently wait for orders to pop up capable of absorbing their sales.
Thank you for the explanation, I did not know that payment gateways actually work in such a way - It did not make any sense to me that they will hold most of coins when in the same time they need to pay merchants in fiat, but their strategy obviously makes sense and makes them profit.
In any case buying things with
BTC according to what you wrote actually has virtually no influence on price, so this is something we can not count on.