All the panels should have about an eighth inch gap. Have him cut a gap where there is none and embed alkali-resistant tape there and thinset it in......unless this is a Kerdi shower, in which case, gaps are irrelevant as the Kerdi will waterproof everything.

I think I had 1/4" gaps when mine was done. I recall because I wondered why and asked. The guy filled them and put mesh, you could not notice.

If they are this big, will water get in? Do I need to have him pull apart and redo?

What happens if the panels rest on each other without a gap.

And if the gap is 1/4", won't the thinset and tape make that a non issue. I would think the thinset and tape joint is actually stronger than the durock itself.

Does this really warrant pulling apart the job to redo the gap??

The reason for wanting a small gap is so that one panel doesn't slightly overlap the lower one, causing the upper panel to protrude slightly. This happens more with the panels that have the rounded side edges. Gapping the panels eliminates any chance of that happening.

bill_vincent and mowers are correct.

mayflower1032 here is the big concern you should have:

There are more than one kind of fiberglas tape in the hardware stores.
If you want to double-check what your installers are doing, please research this thoroughly. I've seen plenty of installers who think that fiberglas tape made for drywall is good enough for concrete. Learn to spot the difference with a quick visual inspection. It is easy to get hoodwinked on that day when they put the tape and then tile over it all in one go. If it's the wrong tape, it could last decades, or years, or.. but you're better off with the prescribed tape. It costs WAY more. Buy it yourself if you want to be sure it goes on your walls. In many big box hardware stores the staff will point you to the wrong tape, as they know nothing. When challenged, they will say it's good enough. I've seen this happening many times. Beware beware beware!

What do the Durock website installation instructions say about the tape?

Alkali-resistant tape is the only right kind. That's where lazarususa is correct.

Whether or not the two panels that touch need to have a gap scraped out, is not something one can ascertain without being there and seeing.

hth

Here is the large gap....larger than the 1/8" the manufacturer recommends.

that gap is OK. It's not a problem. It's within range. Tolerance.