Left grill open in rain overnight

Just take it apart as far as you can and use a lot of paper towels. Some parts are hard to reach, so you will have to improvize.
We've never done that, but my wife likes to clean all the nooks and crannies.

In addition to Gary's advice, I'd remove all the wet pellets immediately. Then I'd cycle though the auger plenty of dry pellets at high heat. Next, and this may be risky so proceed at your peril, I'd consider applying a "very light" spray of food grade oil on some pellets and cycle those through in an attempt to lubricate the internals and prevent rust. Standby ready to deal with any safety issues! And for certain, I'd call Cookshack and get their insight.

Finally, keep in mind that this kind of winging-it advice is CHEAP for good reason.

is the PG1000 really that different from the PG500??

when i left the oven doors open on my PG500, and it rained, certainly no water got near the pellets or the auger.

i ended up with about an inch of water in the area under zone 1 and 4.

Lid wasn't open but ours did go through a pretty intense thuderstorm. Pellets stayed dry and as for the rest of it; since it was wet anyway I sprayed in some degreaser and hosed it out. Let it air dry and no problems at all