An AP close combat weapon and CC skill of 19 mean the Gakis can take out line troopers, but they will have to get lucky to take down any one skilled in martial arts. However, the real reason you are taking the Gakis is for their Explode skill, which triggers when they go unconscious, centering a blast on the Gaki and damaging everything within at DAM 14 shock.
PH 14 is all the Gakis have the keep themselves safe. Ideally they will choose a route towards their foes that avoids line of fire, or passes through smoke. Remember that since they are impetuous, they won't even get partial cover unless in a link team.
Once a Gaki is within 13 inches of its target, it can take a double move and end with the target in the blast radius. If the opponent shoots them, the Gaki will explode, which is essentially why you got them anyway. Keep in mind that Shock will stop this skill from activating, so chose targets accordingly.
Gakis go down to any fire. If you have to wait for them to come to you, set up mines or put models with Shock ammo in a place to ARO them. Shock will prevent them from using Explode. If you have to take them out in assault, use your knife, since it has Shock ammo.
Gakis are cheap enough to fill the last few points in a generic combined force. If you want a super cheap harassing piece, consider their fertile sisters the Preta instead. In the Morat Aggression forces Gakis can only be purchased by purchasing a Oznat. The Oznat also allows them to be linked along with Pretas which can make for a very cheap group of orders to power a harris or other elite Morat models.
Keep in mind the Gaki's extreme impetuous when deploying them. They will be running a minimum of 6" at the enemy every turn, so try to keep from losing them to a free ARO.
Active Turn Role
The Gakis want to get into as large a cluster of enemies as possible and explode. If being brought as part of a link, they will either hang back, or try to stay out of sight as the link moves up to put the Oznat or Pretas to use.
Dodge is the best and only option for the Gakis. They will dodge at 14 and have Kinematica one, allowing them to get to safety with ease. Keep in mind that Kinematica also increases their engage range, and they should always try to engage against a foe foolish enough to walk near them.