Replace "touhou" with the tag you are looking for.
Adding "id:<20000" will show you all the posts with that tag that have a post id of less than 20000. "tag id:<40000" will show all posts with that tag with a post id of less than 40,000 up to the 20,000th search result. This will likely include posts from the previous search. The point being; if you repeat this pattern 86 times, you will have seen every post with that tag regardless of how many there are. Leave out the "tag" and just search "id:<***000" to find all non-deleted posts on Gel. (*** = any multiple of 20 up to 1720.)
There is a faster way but it requires you to pay close attention to the post IDs you are looking at. The post ID is found immediately beneath Statistics on the left side of the post view page.
For example: I want to view ALL non-deleted post that have the blue_eyes tag but there are more than 280,920of them. (The tag counter says 379728 but that includes deleted posts with the tag that cannot be found using the search field.)
First I search "blue_eyes". They are listed in reverse chronological order of posting so ID #s go down as I search through the pages starting with id:1857019 and ending with id:1749882 on page 318. This is a difference of 107137 so I saved myself 5 searches over using the above method.
Next I search "blue_eyes id:<1749882" and I am given the next 20,000 search results. (id:1749877 to id:1640467)
Next I search "blue_eyes id:<1640467" and so on. In this example I would have to perform 15 different searches to see ALL of the non-deleted posts with the blue_eyes tag. The more posts a tag has the more searches have to be used to find them all, e.g. I think touhou requires 16 searches. (Up to 86 if you search for all Gelbooru posts.)
th8827 EDIT: Updated important numbers to reflect new search limit.
Update: (2012-02-19) Updated ALL the numbers.
Update: (2013-04-22) Updated ALL the numbers again.
Wow! Thanks, Jedi. That's what I call a comprehensive reply.
I understand now. And I get the 2nd method too. In fact, using the post ids is pretty smart. I'll try that.
Thanks a lot!
Edit - Just been doing a test. I had forgotten post ids were in reverse order. I realise now that searching for 'id:<20000', then going to the last page takes me to the very first post - Id: 1, so I've got it sussed.