The members in this case are those whom have signed the GATT agreement (last amended 1994). Some of those amendments seem to be directly aimed at the treatment of those outside the union.
In principle it is to reduce friction and issues caused by expanding trade blocks and prevent some of the negative consequences of members that have signed up to this agreement. Which does appear to include the EU.
GATT was orgininally formed in 1947 but following significant amendments in 1980 and in 1994 following the expansion of the EU and similar entities. So the agreement would be bi lateral between the 128 countries that have signed it. Including EU members. A free trade agreement being an overlay to WTO terms. The WTO came into being in 1995 and adopted the GATT membership. GATT members were known as signatories, they are now called WTO members. So the agreement is in force and its terms could be said to be bi lateral as they have been defined and agreed to by the members. Indeed, the 1994 amendment actually clarifies the effect of trade blocks on neighbouring territories and how it should be managed in even clearer terms. Article 24 in its entirety deals with this subject but needed an update in 1994, before the creation of the WTO in order to deal with the spread of protectionist and potentially, destructive trade practices on the rest of the world. It even goes so far as to provide independant means and processes by which tariffs can be set if necessary and ensuring that the recomendation of the working party providing such support must be recognised. See the 1994 amendment and section 4 in particular.
It is not a silver bullet but it does provide for a managed exit under non punitive measures with a defined and reasonable process, if necessary, overseen by a third party acting independantly and with the aim of fascilitating as little friction as possible between the trade block and its neighbouring territory. Fascilitating trade and reducing barriers to trade is the mission of the WTO. Which is why WTO articles are quite sensible and flexible. They also usually have a means of exemption in some cases in exceptional circumstances. This happens in various countries.
This could be a very useful piece of legislation in plotting a sensible way forward. It is well positioned to prevent the sort of thing we are presented with as inevitable, yet it does not quite go all the way there. In any negotiation or legal wrangling, the will of the parties involved still matters. However, it does seem to offer a more positive way forward.