It is technically true to say the the GFA does not prohibit border controls. However the GFA was negotiated with joint EU membership as part of the existing political background, so it was taken for granted.

In practice anything which stops people crossing the border without let or hindrance will be treated by the Republicans as an abrogation of the intent of the GFA.

Border controls themselves aren't really the major item in dispute (and there are already forms of controls for contraband, alcohol/fuel duty, VAT, and movement of people which occur in a dedramatised way) - its alignment of law and regulation which is disputed much more heavily.

This isn't widely understood, I think.

They're not really in dispute right now, but they will be a massive, massive flashpoint following Brexit.

The nationalists will treat it as a symbol, and there will likley be violence associated with any border checks.

If we can keep these away from the border, it would be better, but not sure how you prevent smuggling in that kind of environment.

A soft border with an implicit tolerance of smuggling at a certain level (which in reality is what we have today vs the rest of the world - witness the amount of fake-certified and dangerous goods that come in from eg China with no real enforcement), together with cooperation on enforcement for the bigger offenders (eg the UK assisting in stopping goods before they reach the border if they are destined for RoI and known not to comply with EU standards) would seem to be the pragmatic solution. I don’t think the two sides are really all that far apart on this point.

As I mentioned, the much bigger issue is regulatory alignment more generally for reasons which are not related to border infrastructure (North-South cooperation and the all island economy), where there is a much bigger gap between the sides.