Hi, I'm a UK Doctor. I'm confused by the question here.

In the USA while you use GPA for general degrees, to apply to the Foundation Programme you need to hold a medical qualification/degree - you have to be a medical Doctor to be eligible as it's a post-graduate medical training programme (Equivalent to internship). For US students that means you have to have an MD or equivalent (e.g. DO?) from a recognised Medical School as defined by the UK's General Medical Council

It is your MD or DO qualification that NARIC can issue a statement of comparability on to aid the Foundation Programme application process. The GPA of your pre-med degree shouldn't matter. However if you want to list it to aid scoring, you should be able to list it as just Pass - I did a Bachelor of Medical Science here in the UK but it did not use the 1st, 2:1, 2:2 system for it; it was just Pass/Fail.

I suspect you're being lead all round the houses because the people you're talking about don't understand the difference.