The GTS serves two functions. First, it will definitely allows you to purchase any new Combat Tactics you want, which will unlock bonuses for your whole squad, such as bonus experience from kills, extra item pickups, and — critically — increasing the size of the squad you can deploy on missions. For this alone, the GTS should be an early priority to build. Its secondary benefit is also extremely useful, however. Rookies can easily train here to take on any new class of your choosing, rather than just by random assignment from leveling up in the field. These will lets you proactively fill any staffing holes by training whatever class you need the most. So please be sure to always have someone training even if you have rookies, because it is essentially a fairly free experience.
Advanced Warfare Center
The primary effect of the AWC is that it tends to decrease the healing time of your injured soldiers when staffed, which can be very critical for maintaining a viable, field-ready squad at all times. Its second function is a little less clear, but can be game-changing. After the AWC is built, every soldier is then secretly assigned to a rank at which they will acquire a random out-of-class skill along with their standard choice. Soldiers already past this rank will not retroactively acquire the additional skill, so building the AWC as early as possible ensures that the maximum number of soldiers will receive their bonus. Like the GTS, the AWC offers two useful functions that you want to utilize very early and often to get the most bang for your buck, so it should likewise generally be a number one priority.
The Workshop will essentially give you just two engineers for one. Staffing it will also provide two Staffing GREMLINs that can act as engineers in rooms adjacent to the Workshop. To ensure the most flexibility, you will also want to build it in one of the two central rooms. So please note that it only works with vertical and horizontal adjacency, and not diagonal. You also can only have one Workshop at a time, so you can’t build two and then have them staff one another, unfortunately (although mods will almost certainly change this at some point). Fortunately you can upgrade it to allow another engineer to operate two more GREMLINs. Be sure to surround it with rooms you will want to staff with Engineers, such as Resistance Comms, the Psi Lab, Power Relays, and the AWC.
A straightforward and very essential function of expanding your base, each facility has a great power requirement that, in total, must stay within the limits of the Avenger’s capacity at all time, which is boosted primarily by building relays. Your starting power capacity will only support a few facilities, so an early Power Relay is almost always very important, barring a lucky continent bonus or random event reward to boost your capacity. Please carefully Mind your resources, plan ahead, and build Relays proactively, rather than in response to your need, in order to avoid a situation where a lack of power acts as a bottleneck to your overall progress. You need to build on top of Exposed Power Coils whenever time it is possible for maximum efficiency.
Resistance Comms Unlocked early in the course of the story, this is a critical facility that will lets you maintain more regions on the map as part of your rebellion, which in turn increases your monthly income. Accordingly, you will also want to build this early to expand your network as quickly as possible, because that early income will add up in the long run. Upgrade and staff it proactively so you aren’t able stuck unable to contact a new region with an important Avatar facility to raid, for instance. Proving Ground This engineering facility will lets you build special projects and unique equipment, sometimes with a bit of randomness. It becomes available early in the story to facilitate constructing the Skull jack, and then allows you to also build things like specialized armor or randomized special ammo and grenades that are unique, rather than being available to all soldiers at all time like primary weapons and armor once constructed. These items are extremely useful in the long run, but require materials that are not plentiful early on, and their effects don’t snowball as much as other early infrastructure like the AWC and GTS, so it shouldn’t be quite as high a priority unless you’re especially eager for powered armor (which is admittedly cool).
Scientists, rather than engineers, can be really staffed here to boost Dr. Tygan’s research. Scientists currently have no other use, so building and upgrading the Lab relatively early can easily accelerate your research quite a bit in the long run.
This is the most niche room available, and thus shouldn’t be a priority unless you know a UFO chase is imminent from Dark Events. Even then, the best course of action might still be to lay low and continue building what you would otherwise.
This is primarily just for story-related research projects that bring you toward the endgame, so when you choose to build it will largely be dictated by how advanced you’re willing to let Avatar get. It has a useful secondary effect, however, of letting you know what enemies you will face in upcoming missions, which allows you to kit your team out accordingly. You can load armor piercing ammo or EMP ammo if you will be facing a lot of robotic foes, for instance. Note that conducting Shadow Chamber research projects suspends any other active research, since Dr. Tygan can’t multitask.