Maybe on the gate is possible but it's easy to donate via the PCGB site, you get an email confirming your donation which you print out and use to get in
https://www.porscheclubgb.com/news-and-events/events/2018/october/porsche-charity-day-at-goodwood
https://www.porscheclubgb.com/news-and-events/events/2018/october/porsche-charity-day-at-goodwood