The GFA is relevant as it is an agreement with a member state so will obviously impact on relationships post exit
But not an agreement with the EU. Remember that the EU is not an individual country, but a name for a collection of independent countries that joined a common club.
I recall that you corrected me when I referenced by mistake that the MEP Meiread McGuiness was from UK by saying that Ireland is an independent country.
A contract can only exist between two parties or entities. EU is an entity of its own. The 28 members have agreement with EU as a single entity. The EU draws up the rules for each of the members to follow. If members make separate agreements with each other that is their private business. Such agreement does not flow into the EU as part of EU law.
Example
A company, EU, employs three people; Engs, Frog and Germ. Each of the employees have a contract with their employer. If any of them want to resign they serve notice to EU in accordance with EU rules they as individuals agreed.
If Frog has made a deal with Germ outside of work, the employer, EU, is not interested as not party to the deal.
So if Frog resigns from EU, Germ can't say to EU it is not allowed as Frog has a deal with me and if Frog leaves EU I might be disadvantaged.
Easy when you think about it logically and rationally as opposed to subjectively and emotionally.
My comments wrt Ireland refusing to continue discussions until the GFA issue was sorted referred to the discussions post invocation of A50
The withdrawal briefing document 7551 was issued by EU in Jan 2017 was different to what they issued in February 2016. So EU moved the goalposts after the referendum. Bit naughty that don't you think?
The EU withdrawal briefing issued in January 2017 was two months before UK invoked Article 50. So even before discussions began EU had decided on a 2 phase approach of WA first and trade second even though Article developed 8 years earlier said otherwise.
So how do you explain that discussion was stalled post invoke A50 to separate WA and trade when EU had decided that was their preferred procedure (against UK preference to conduct both at the same time in accordance with Article 50 agreed in 2009 and the EU briefing note issued in January 2016 before the referendum) before any discussion began?
If there is an agreement that impacts on future relationships between the leaving party and the EU or any member of the EU, then of course it must be considered in any discussions on future relationships
UK is leaving the EU as a single entity and not 27 separate countries who have their own individual contracts with the EU as a single entity. GFA is not written into EU law remember.
The EU can decide on red lines that will impact on deals, in the same way the UK can and did
EU has red lines for accession to the EU (standards, codes, security, etc) and applications to become a member have to be accepted by all existing members.
There are no EU red lines for withdrawal from the EU detailed in Article 50 and withdrawal by a member does not have to be approved by the other members.