What is MG?

“How long did it take the MG to spread?” John asked.
“Not long. The Marauder was on the ground in less than three minutes.”
 
This is an excerpt from page 118 of Alien Earth: Hope Cometh, the first time MG is mentioned.
I won't go into great detail as to what it is, but just because you aren't given an explanation right away doesn't mean you'll be left in the dark.  You will learn what it is and where it comes from later on in the book.
 
If you can't wait, however, here are all the pages MG is either mentioned or explained:
 

• Page 118 (mentioned)
• Page 146 (explained)
• Page 224 (mentioned)