And D-Day Dice, too!
I'm an "A"vatar.
MGF rolls are done during combat - page 12, the very first paragraph:
"The number of Soldiers Units lose is equal to the Defense (DEF) of the Sector (plus Machine Gun Fire if applicable)."
The die roll is not explicitely defined in the rules, but we were playing that you roll it every turn.
Yes, you roll MGF during the combat phase. If you stay in the same sector more than 1 turn, then you have to roll MGF every turn.