Interesting question. Three possible answers.
1. The Govt is determined (broadly) not to ask *anything* from the EU so as to minimise EU leverage. (The damage to various key UK interests that that involves is apparent here.)
2. The Govt thinks the UK can essentially free-ride on the EU procurement regime (eg UK companies can set up EU subsidiaries entitled to benefit from the EU regime, which is unlikely to change.)
3. The US on the other hand could be asked to open up sub-Federal procurement, and to scale back on “Buy America”. But good luck with that, given the US Trade Representative’s position. 
Note that Cummings has a set of views in the area, as @davidallengreen pointed out. ft.com/content/67a721…
NB too that Johnson fought the election on the basis of the political declaration, which said this: “beyond ... the GPA” has been dumped.
