Glenn's Gases, the Gases Framework, and the Gases Framework API are all distributed under the GNU Lesser General Public License (LGPL). In short, this means that you can freely distribute and alter the mod as long as a reference to the original remains. External modifications such as MCTweaker tweaks are also okay.
TL;DR for modpack makers: You can legally include Glenn's Gases and the Gases Framework in your modpack without explicit permission as long as you have a link to this official page somewhere in your documentation.
Still, if you are including Glenn's Gases and/or the Gases Framework in a modpack, it would be nice if you let us know. Drop an e-mail to firstname.lastname@example.org.