The biggest problem is that once you hit a spotter, you're spotted. No matter the distance, cover or lighting.
It's also true that once you kill someone, or get seen even once, all aliens enemies have a number of turns (depending on their 'intelligence' stat) where they know your location. That includes being seen by your own mind-controlled units, or having an enemy expire from wounds/flames three turns after the fact.
But being 'seen' is not being 'spotted'. They know where you are, and might move closer, but they can't snipe you.
What exactly is going on with the Gangers, I don't know. Most likely, you're misjudging their vision or who exactly is shooting at you. Your agents aren't lit up in any way, are they? Flares in the 'trunk', is there perhaps still an armour with lots of personal light, etc? I tried a quick test battle with a helicopter and a RD outpost, and all the Gangers were just milling around near my landing site, one even had a visual but decided to come closer to melee. The ranged fire was from either Sailors or Pioneers, no exceptions.
For a more precise answer, you could post a save with the battle and say which enemies are shooting at you that you think should not be.
Of course, if you're playing BOXCE, all bets are off.
X-Com and by extension OXCE aren't really good as or designed to be stealth games, and modders shoehorning whatever tools they're given to fit their vision doesn't always produce particularly transparent gameplay effects.
I'd attach a save, but I didn't save the actual start of the battle and loading from the Geoscape seems to be giving me an entirely different seed. I wasn't using any lighting and all of my guys were using kevlar vests. So I guess it will always be a mystery why the gangers seemed to be shooting at me from beyond visual range.
So, to clarify, if I hit an enemy with the spotter attribute, does that mean that all snipers can now shoot at my soldier regardless of any other circumstances? But if an enemy just dies, this does not enable enemy sniping (just that they might move towards my location)?
Is my soldier who hit the enemy the only one who can be seen, or are all of my soldiers now seen?
If I shoot at the enemy but don't hit them, does this trigger the spotter/sniper mechanic in any way?
Are there any other circumstances under which an enemy can snipe me, aside from hitting an enemy?
If you want any consolation, the red dawn outpost mission can be one of the trickiest ones in the game (if you play ironman and higher difficulties). This is because you usually have only access to helicopter and so few troops and you might not even have access to grenades (to avoid sniper/spotter) unless you have been successful in getting all the cult networks and other requirements of promo II. (You definitely shouldn't even try to complete it successfully unless you have grenades or you spawn in a city where you can use the cover of buildings.)
Many times I've just decided to immediately abort the mission and wait for a better spawn. Also many times if a sailor has spawned nearby, I've just focused on capturing it, getting back to the helicopter and running away.
This seems like this would be good advice for me to follow, but I'm terribly pig-headed; I'll keep going to Exobiological Contamination missions even though I know darn well that nothing good is going to come of it. So I'll most likely keep giving the Red Dawn Outpost missions my best shot.
stay in the dark and Throw grenades they don't trigger the goofy sniper/spotter mechanic. try placing snipers far away as possible if you do shoot to avoid getting shot. Kill off key targets and avoid shooting to much with the snipers they will inevitably get shot. everyone and their mother is a sniper on red dawn besides the lowest rank guys. So don't bother with smoke. make a ring of light around lz and never leave it stay in dark.
use riot shields with frags until you get your hands on heavy tac vest. then you wont have to play so careful. I usually take down red dawn first Because I just hate the play style you need to fight them them without heavy losses.
Sniper/Spotter is basically an AI cheat. If you shoot anyone or get seen who is a spotter the snipers can see you no matter were you are. for a turn or more depends on enemy. they will shoot you through walls if thin enough farm (mib missions). they will also throw frags (if spotted) if they don't have a clear shot at you so try and hide under something or use a wall to avoid that they still know your location but at least you don't get frag thrown at you.
You will recognize who has sniper/spotter after some time and learn to play against it good luck.
I haven't gotten to Promotion II; is there any easy way to get them in the meantime, other than sheer luck? I've tried melee, too, but it seems to be an even worse idea than shooting the enemy.