Come on OP, dig around in the drawers for the big girl pants for both you and your daughter and you WILL find solutions which are SAFER for other pupils arriving on foot using this gate you turn in (which the school has gone to some time & effort to procure & affix not just a sign, but a BANNER - as you described it - asking parents, ALL parents, not to do this). Your diagram didn't include the pedestrian walkway & zebra crossing you mentioned (or the position of the enormous banner asking you not to turn there), so all we have to go on is that the school is quite certain they do not want parents turning cars in the gateway, but you & ~10% of other parents STILL feel special enough to ignore this.
These solutions which are SAFER may take you more time, they may require hybrid means of transport, they may involve experiencing weather, they may involve walking further than a couple of hundred metres. But they won't involve ignoring the school's safety requests not to turn in their gateway or neighbouring business' car parks.
DD is either eligible to access help for transport to school from your local authority (living 6 miles away) OR
- she's at a private school and you don't live on one of their bus routes
- she was eligible for a nearer state school but you opted to send her to this one because it was in some way more beneficial to you
- you've been offered a council transport option but don't find it convenient so drive her in yourself
which of these is it @NortonBuns ?
If eligible, get the application in & find out where your nearest bus stop is to drop her, so she can get to school the same way as the other 90% of pupils & arrive at the main gate. https://www.gov.uk/free-school-transport#:~:text=Children%20of%20compulsory%20school%20age,child%20is%208%20or%20over
Otherwise, the A road you approach from is only 2 miles from the gate, and high school pupils are expected to walk up to 3 miles each way before qualifying for free transport (per link above). So I suspect you are probably not casting the net wide enough for places which are safe for you to stop & return home, and still within what is considered a reasonable distance to walk to school at her age.
It feels like you are deciding it's unreasonable for her to walk 50 minutes in in the morning, when it's a national policy that it's perfectly reasonable. By extension, if you still dropped her to the gate but didn't turn, I'm sure it wouldn't take you fully 50 minutes to get out if continued via the main gate to return home.
It's just INCONVENIENT.
The school has been quite clear they don't want your convenience trumping pupil safety in the morning. Even if it adds time to you or DDs daily trip in. Unblock your tin ears & find a solution!