I never used that informative Annex G of EN 1591-1. It has no correspondent in EN 13445-3.

I can give you just some speculations on the subject, not a documented engineering answer.

1. In the beginning of Annex G there are some explanations. Some gaskets have an outer spacer-ring in the form of a solid metal annulus (or similar 'rigid' material) which is located between the outer periphery of the sealing element of the gasket and the bolts. Spiral-wound gaskets often have an outer spacer-ring of this type.

The bolt load applied at assembly may be sufficiently high to seat the flange faces on the outer spacer ring of the gasket. In this case the flange faces, being (slightly) inclined, make initial contact with the outer periphery of the spacer ring.

In my understanding F_B(0*) is a "limit" force calculated on this bases (and as you can see, I prefer to use "_" instead of subscript).
The relation (G.1) "seems" to be a geometrical condition. By one hand there is the effect of flanges rotation at distance (h_G-h_D) and the (elastic) stiffness of the gasket. By the other hand at the outer edge of spacer there is a change during "bolt it up till spacer ring contact", as difference between e_G(0,0) i.e. the initial thickness of gasket and e_SR (spacer thickness).
So, if my interpretation is correct, h_D must define the outer periphery of the spacer ring.
I would imagine it is an arm defining the outer edge of spacer ring, considered in the same way as h_SR is defined (i.e. measured from the outer edge of spacer to F_B). In fact we may consider that the contact between spacer and flanges is just the beginning of the existence of F_SR, so -as definition- h_D would be similar to h_SR.
Again, this is just my speculation! What about your opinion?

2. About m- just another speculation. "m" is "crying" it is the "gasket compression factor" as it is (still) referred in EN 13445-3 Appendix G, which claims it is based on EN 1591-1. Reading G.9 of EN 13445-3 you can find out some details and values for "m". However, it is clear also that "EN" prefers to work with the "minimum" stress on gasket rather than with minimum "m*p" stress on gasket and that’s why EN 1591-1 tries to eliminate references to "m". In fact EN 1591-2:2008 is focused on this approach, giving you directly "minimum required compressive stress" Q_min[L], Q_Smin[L]
Returning to EN 1591-1 G 3.4.4 "Maximum internal fluid pressure" for me it appears that there EN was "forced" to consider an "m" as a part of non-dimensional parameter, because it was difficult to further complicate that subclause (the alternative would be to force a complicate equation working with Qmin/p instead of "m", but here EN preferred to maintain a "simple" relation). However the price is now a strange "m" appears there!

If you don't want to have this mix (as "m" imported from 13445-3)you may consider there is a note that says " This subclause [G.3.4.4] may be avoided. The next subclauses contain the same information in a more general way, but it does not clearly show the visible effect of the upper limit". Hmm, "clearly"....funny, isn’t it?

My best regards.