It seems that the "Sans" font using in GIMP 2.8 is not the real "Sans" font.
No, it isn't (if you're referring to a font called 'Sans'), and not the family. Sans, Serif and Monospace are mapped by fontconfig to different font on different platforms, they are supposed to use one font from those families.
When I used GIMP 2.6, it mapped to "sans-serif". Now, GIMP 2.8 mapped to "serif". How can I change it back to "sans-serif"? Thanks for your attention.