Lex The GFA never even considered that the UK would leave the EU, because back in the 1990s it was not within the realms of serious politics
The GFA made the border invisible, so that NI Nationalists could ignore the fact they were living in the UK and not a united Ireland.
It was a major reason for NI Catholics and the RoI voting so heavily in favour of the GFA in the GFA referenda.
A hard border, even just for goods, would destroy the spirit of that consensus.
The RoI, as part of their duties under the GFA, even changed their constitution, to no longer lay claim to NI, but to accept it would only unite with the RoI if NI chose in a future referendum to do so
There are at last count 143 areas of cross-border cooperation, which might be hindered by a harder border:
https://www.rte.ie/news/brexit/2018/1119/1011801-brexit-northern-ireland/
EU Ombudsman Emily O'Reilly has called on the European Commission to publish a key document
spelling out all the areas of North-South co-operation under the Good Friday Agreement that are at risk because of Brexit.
The confidential document deals with the so-called mapping exercise,
which became a key plank of the Irish Government's strategy to highlight the risks to the Good Friday Agreement.
The mapping exercise highlights the extent to which North-South co-operation relies upon, or is enhanced by, mutual EU membership by both Ireland and the UK.