[dragonne]

The SIL Open Font License restricts subsetting, so you do need to be careful when doing this (the license requires that you change the font name if you reduce the font's display quality). Google has gotten waivers to allow them to alter the fonts without changing their names.

The repository with the fonts and tools is here: https://github.com/google/fonts

[khaled]

Only fonts with “reserved name” clause have such restriction, and then you are only required to rename the modified font.