GDP is value added. If there are less people working, I'd hope that value added would go down.

What about a situation where fewer people are working more productively?

Wouldn't that be orthogonal to basic income? i.e. You'd get that anyways so it would cancel out?

No way to know without looking at the data, but I can see a few methods where a basic income would increase productivity.

One talked about in the article is that workers who choose to work, tend to work harder/more productively than workers who are forced.

The whole point is almost moot though because the data in basic income experiments show that employment rates tend to go up not down.