How to get bubble on troops gathering resources?

It is called a troop aegis, and it can only be gotten as a super rare random drop or bought with real money currently.

... and while the troops are protected all that they gather is not! So if you see someone coming to attack them, send them home with what they have gathered so far IMMEDIATELY.

It seems to limit the usefulness of the buff, but then again, it's nice they are all not in the hospital or dead when you return to the game.