How are g forces measured?

When we say "number of g forces" the g we are typically (not always) talking about is the average overall Earth of around g = 9.81 N/kg or m/sec^2 (See NOTE)

So, for example, if a jet jock pulls 2 g's in a hairy vertical climb, he's pulling 2g = 19.62 N/kg on his body. And if his mass is m = 100 kg, that means his effective weight is W = m(2g) = 1962 Newtons.

Bottom line. g forces are measured as the number of average g's.

NOTE: N/kg = kg.m/sec^2//kg = m/sec^2 which is to say they are equivalent units.