#CJam, 35 27 bytes
ri{_e-3_i}g;1mOo]," kMG"='B
Thanks Dennis for removing 8 bytes.
This doesn't print .0 in the online interpreter. But as Dennis has pointed out, it works fine in the Java interpreter.
###Explanation
ri e# Read the input as an integer.
{ e# Do:
_e-3 e# Make a copy and divide by 1000.
e# This will generate one more item in the stack for each iteration.
_i e# Make a copy and truncate to integer.
}g e# until the integer part is 0.
; e# Discard the final value with integer part 0.
1mOo e# Output the number before it with the correct format.
], e# Count the number of iterations - 1.
" kMG"= e# Select a character according to the number of iterations.
'B e# Output B.
