You should be calling out an immersion gold over electroless nickel
if you plan to solder the BGA. gold thickness should be called out 2-
8 microinches thick with the nickel at 100 microinches thick
minimum. Gold Body typically refers to the an electrolytic gold which
isn't the first choice for soldering to. It can have thick deposits of
gold which can create soldering challenges.

The silver filled vias can be called out "vias defined by (filled via
layer) are to be conductive via filled with CB100 or equivilant." You
should provide a gerber layer that defines exactly which vias are to
be filled (name it: via fill layer).
I hope this helps,

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