The missing character is M.
The sequence is a list of letters that are two places apart in the alphabet, starting with A. The next letter in the sequence is M, which is two places after G.
A. A is the first letter of the alphabet, so it is not two places after G. B. G is the seventh letter of the alphabet, so it is not two places after G. C. M is the thirteenth letter of the alphabet, so it is two places after G. D. P is the sixteenth letter of the alphabet, so it is not two places after G.
