Note: this page needs to be cleaned up as the images given are confusing. Official forum post here has better images & details the history of the Gang Up bonus.
The Gang up is one of the most important tactical features in UniWar. If you attack the same enemy unit with multiple units, each subsequent attack will be more powerful than the original. (Note: this does not necessarily mean it will be more damaging, even if using identical units, due to the randomly seeded damage factors.) The power of the second attack depends on the position of your second attacking unit relative to the cell of the first attack.
This bonus will be lost after you end the turn or after attack another unit, but will stay if you produce new units or just move your units around the map. In other words, the Gang Up bonus works even if the two attacks were interrupted by any other action, as long as they are not attacks to other units. Gang Up bonus counts relative to the previous attack, so if you attack an enemy with multiple units, Gang Up each time will be calculated based on the previous attack. It is not cumulative over multiple attacks.
The Gang Up bonus gives the attacking unit a power up of +1, +2 or +3 to attack. If the first attack is ranged, the bonus will be +1 regardless of the position. If first attack was melee (close combat / adjacent cells), the bonus will depend on the position of the second attack.
Determining the precise bonus for a melee attack can be complicated. In the image above, the green cell represents where the first attack is made from, and so any of the orange cells can be where the enemy unit is positioned. So let's look at the bonuses given to the second attack.
- +1 — second attack made from one of the orange cells.
- +2 — second attack made from any of the white cells, or the green cell. (Yes it is possible for a follow up attack to come from the same position as the first attack. Helicopters and speeders both have move after attack. Marauders can attack in their first move and then leave. And sometimes it is the right tactical decision to start an attack with a weak unit which dies, leaving the green cell vacant.)
- +3 — second attack from any red cell.
IMPORTANT: The second attack doesn't have to be a melee attack, it can be ranged. The Gang Up bonus depends only on which cell the unit attacking second is in! If you use this rule properly you can significantly power up your ranged attacks.