Gotcha Again has evolved from the few basic indicators of the first few versions, so this thread will show examples of the currently included ones and explain what triggers them:

Unit indicators
Shootable: There will be clear LOS to a unit and that it is within range to be shot at the new location. Screenshot
Flanked: In addition to a unit being shootable, it will also be flanked at the new location. Screenshot
Squadsight: There will be clear LOS to a unit at the new location, but it is only shootable because of the Squadsight ability. This is only shown when moving a unit with the Squadsight ability. Screenshot
Flanked Squadsight: In addition to the unit being shootable because of Squadsight, it will also be flanked at the new location. Screenshot
Hacking: There will be clear LOS to a unit that is not within range to be shootable at the new location, but it can be hacked or damaged using one of the following hacking abilities: Combat Protocol, Haywire Protocol or Full Override. This will only be shown when moving a having one of those abilities. If the unit also has Squadsight, the Squadsight indicator will be shown. Screenshot
Friendly Spotted: This indicator will be shown for friendly units that there will be clear LOS to and will be within the moving units own sight at the new location. Screenshot
Spotted by Enemy: When moving a VIP, this indicator can be enabled to be shown for enemy units that will have vision on the VIP at the new location. Screenshot
Lone Wolf active: When a unit with the Long War 2 ability Lone Wolf, previews a move to a tile where this ability will be active this is indicated next to the moving units healthbar. Screenshot

Objective indicators
Shootable: For VIP objectives that should be neutralized or other objectives that should be destroyed, the icon on the arrow pointing at it will change to indicate that there will be clear LOS and that it is within range at the new location. Screenshot
Flanked: For VIP objectives that should be neutralized, the icon will change to indicate when it will also be flanked at the new location. Screenshot
Squadsight: For VIP objectives that should be neutralized or other objectives that should be destroyed, the icon will change to indicate when it will be shotable at the new location because of Squadsight. Will only be shown for units that have the Squadsight ability. Screenshot
Flanked Squadsight: For VIP objectives that should be neutralized, that are shootable because of Squadsight and will also be flanked at the new location the icon will change to indicate this. Screenshot
Hackable: For objectives that should be hacked, the icon will change to indicate when that is possible from the new location. Screenshot
These icons can vary depending on the iconpack used.

DoorHacking indicators
If a door will be hackable remotely using the Gremlin, the vanilla indicator, that normally only appears when previewing moves to tiles next to the door, will appear. Screenshot

Indicators ADVENT towers
Regular icon: When an ADVENT tower is first revealed by a friendly unit, this icon will appear pointing at the tower. Screenshot
Hackable: When a unit able to hack remotely will have clear LOS to a tower and be in range at the new location, the icon will change to indicate that it will be hackable. Screenshot
Never hackable: Once a hacking attempt on any tower has been made (whether successfull or not), the icons for all towers will change to indicate that they can no longer be hacked. Screenshot
These icons can vary depending on the iconpack used.

Indicators along the movement path
Overwatch triggered: When previewing a move that will trigger an overwatch shot from a hostile unit, the tile that the shot will trigger on will be indicated and the overwatch indicator for the unit in overwatch will change to indicate that it will be triggered. Screenshot
This indicator will also show if you are moving a suppressed unit that will trigger a shot from the suppresser.
Pod activation triggered: When previewing a move that will trigger a pod activation on an already visible pod, the tile that will trigger the activation will be indicated and the unit(s) in the pod that will spot you on that tile will be indicated by an icon. Screenshot
Noise generated: When previewing a move where noise will be generated from kicking in a door or breaking a window, this will be indicated where the the move will generate the noise, if this feature is turned on (the default is off). Screenshot
These indicators will also be included in the "summary" indicator at the destination tile, while the next two will only be shown at the destination tile.
Smoke: When the destination tile is covered by smoke from a smoke-grenade, it will show a smoke indicator on it. Screenshot
PsiBomb: If the destination tile is within the area of effect from an active PsiBomb, this will be shown by the PsiBomb indicator. Screenshot.