Thank you for the feedback, @homeexchange!
We’re going to be looking at adding gallery reordering to the block editor in WordPress 5.1, you can follow progress over on this issue.
In the mean time, you’re still able to reorder galleries by clicking on the “Edit Gallery” button at the top of the gallery block.

The gallery editor “pencil” appears with the gallery box in my post, not at the top of the page and it is what I have always used to edit the gallery. Until the update to 5.0, I could rearrange sequence by dragging and dropping on my iPad. Now, nothing will move position. I can still add and and delete pictures, but sorting seems to be impossible although it says at the top of the page to drag and drop to sort. Could this be an iPad problem? As I’m on the road it is the only way I can access the internet.
Thank you for the extra information, @homeexchange.
It does appear to be a mobile/tablet problem, I’m able to reproduce the bug you describe on my Android phone.
I’ve reported a bug to get this fixed: https://github.com/WordPress/gutenberg/issues/12965