The UK are creating conditions that force a border
Brexit negotiations are between two parties, EU and UK. If negotiations do not produce an agreement then both parties have failed. If EU bothered about GFA why was it not included in Article 50 which was developed 20 years after GFA?
Your logic is like saying an unwanted pregnancy can only be one person's fault. Totally overlooks fact that end result was the action of two parties.
Who puts up the border us irrelevant. The UK have a responsibility under the GFA to ensure the border is kept open
In the event of a no deal all legal agreements between EU and UK cease to exist. So what EU does afterwards is beyond UK control. EU regulations require border control between EU and Non EU countries. Does the GFA force the EU to ignore their rules? Can the UK, after it had left the EU, force the EU to ignore their rules?
Your logic is like saying that someone who resigned from their job is responsible for what their former employer does afterwards.