There is absolutely no way D.Gray-Man is a million seller, maybe ~700k-800k max
Here is the "known" sale since 2008 (around the time of her injured wrist).
101,918 *,442,961 D.Gray-man vol.17
156,739 *,400,788 D.Gray-man vol.19
041,141 *,382,501 D.Gray-man vol.20
148,236 *,363,413 D.Gray-man vol.21
057,539 *,352,866 D.Gray-man Vol.25 - latest volume
288,592 939,409 Hunter x Hunter Vol.33
D.Gray Man has 22,500,000 million copies in print as of volume 24, 60% consider sold so around 600k-700k more or less.