Runner, look into an ice vest. I have one called FlexiFreeze. With it I can be outside in hot weather for reasonable periods (barbecue anyone?). If I'm not too active I've been out for a couple hours in up to 90 degrees (in the shade). I've mowed my lawn with it (small lawn, but a non-motorized reel mower). I've even played golf in it. It would be great for running. I find I have to put it on for 10 minutes or so before I go out, and keep it on afterwards until I'm completely cooled down. Then a cool shower with no soap is nice (important not to disturb your skin microbiome).

My GD is waste to shoulders all around, mostly I've only reached red welt stages with some blistering and a few mall scabs. It itches all the time, like a 5 or 6 on the itch scale even when it's barely visible, but it gets really intense when it flares up.

Without the vest when I do go out on warm or humid day I get prickly and my spots get red and enlarged quite quickly. When I wear the vest they don't may get prickly but they don't progress. If I didn't wear the vest I'd suffer for days.

That's all preventative, but sometimes it just the GD just rears up and in these situations the ice vest is soothing and significantly minimizes itching and inflammation.

I spend a career in toxicologic and pharmacologic research and I come away from that experience not wanting to ingest or slather unnatural materials on my skin, especially to compromised skin (we're finding out more about adverse effects to the skin microbiome all the time). I've written about this ice treatment before, but no one responded either way so I don't know how it works for others. It has allowed me to completely stay away from any ingesting and slathering, etc. I see it as a means by which I have some control over my GD and not it over me. Try it, you'll like it.