Mechanisms are bathed in a gel with an extremely low friction coefficient. Unfortunately the gel congeals at a high rate actually making the mobility worse until it is flushed from the system.
Gel Injectors is an augment which increases an agent's base AP by 3, but then reduces their base AP by 1 at each alarm level. The penalty is uncapped, so the agent's base AP continues to be reduced even after alarm level 6, eventually reaching 0 AP if the agent stays in the mission long enough.
If Gel Injectors is installed during a mission, the agent's base AP is immediately increased by 3, regardless of the current alarm level; the penalty only applies to alarm level changes which happen while Gel Injectors is installed. Gel Injectors is the “opposite” of Reactive Myomer; if both are installed on the same agent, the combined effect of both augments is +1 base AP, with the alarm level penalty and bonus cancelling out. It is not possible to install Gel Injectors more than once on the same agent.
Gel Injectors is a risky augment because the alarm level penalty is uncapped; if the agent stays in the mission too late, their base AP can become very small or even zero. If this happens, the agent can only move if they get additional AP from a Stim, another augment, or some other effect.
On the other hand, if the alarm level is not raised for other reasons, then it takes 20 turns before Gel Injectors actually has a negative effect, and 35 turns (5 turns into alarm level 6) before it has taken away more AP than it has given. If you can consistently finish a mission and leave before alarm level 4 then Gel Injectors is quite good, especially since it can help you finish missions early. The problem is that if something goes wrong then you may not be able to leave early, and then Gel Injectors is actively harmful.
Gel Injectors may be more useful in late Endless mode, where a well-equipped team can beat most missions in just a few turns.
As Reactive Myomer is generally considered to be a good augment, the fact that Gel Injectors does (almost) the opposite suggests that it is at most a situational augment which can work well in specific strategies or play-styles. The community has differing opinions, ranging from “Gel Injectors is a good augment” to “it's worse than an empty augment socket”. It is inadvisable to install both Gel Injectors and Reactive Myomer on the same agent, as this gives only a flat +1 AP — a dismal bonus for two augments.