This is a pretty cool idea but it feels really bare bones at the moment. We have no idea what his passive does to empower his next ability, what his w duration is, and what his cooldowns are which are pretty important since he wants to stack his q and w on targets. Also, his q seems like it would be kind of boring as a constant heal over time.
I'm ignoring the specific damage and healing values you gave since they scale weirdly with level and seem way too high though it's hard to judge given the problems listed above.
His e is confusing since we don't know how much it increases damage by or if the heal when hit portion can proc more than once. If it can't then you waste the % missing health healing since they won't be low from the first hit. If it can then it would be ridiculously overpowered, especially against any sort of dot or multihit attack. Either way, a damage amp for an ally makes him want to use it as often as he can but the way the healing works makes him want to save it until they're low on health and being attacked. For the enemy, the damage amp says that they're supposed to kill the person he used it on but the heal punishes any attempt to do that. In general, unless the heal is ridiculously overtuned, the enemy won't bother playing around it outside of maybe landing phase since 3 seconds is way too long for it to be worth waiting out.
The ult is unique but unless his w damage is high enough to kill people with just that then all it would be good for would be protecting him from divers or assassins.
Also, I doubt alternate builds would work with this guy outside of maybe an ap heavy build in the jungle due to only having one damaging ability, no reason to build tanky, and no waveclear.
Here's something I threw together for fun that keeps the idea of a stealthy support and hopefully improves the design while hopefully making it healthier and better able to function in other positions.
His abilities would all be shorter ranged with auto attacks similar to Rakan or Thresh in range. The idea behind this would be to make his stealth matter more since with his current kit, he'd usually be far enough out of harms way for the stealth not to matter. Overall, he would play closer to Raman and maybe Pyke.
Passive: He stores up to 3-5 wisps at a time (levels 1, 6, and 11) that he uses as ammo for his q and w. He regains one wisp every 6 seconds. Auto attacks decrease this timer by 1 seconds, increased to 4 against champions or large monsters. It would probably need some mechanic to let him lower the cooldown while playing as a support but I'm not sure what the best way to do that would be.
Casting abilities on allied champions other than himself gives him a 2 second burst of decaying movement speed while moving towards enemy champions and casting abilities on enemy champions gives a 2 second burst of decaying movement speed and bonus range on his next auto (total of 450). Each effect has a static 5 second cooldown and resets the cooldown of the other. Movement speed boosts scale with level and let him ignore unit collision.
Q: Unchanged for the most part. Range is around 300. Costs one seed to use along with mana and has a static 2 second cooldown scaling down to 1 second with ranks in r. Gives 20% tenacity to the target if they have at least one stack for some extra utility.
W: Same treatment as q with range, charges, and cooldown. Dot lasts 4 seconds and stacks up to three times. Now slows by 25% for 1 second upon application to help him hit and run. Applying the third stack and each thereafter deals a small burst of initial damage. Deals the same damage over half the duration against minions with an additional 50% dealt as initial damage.
The passive, q, and w would make his playstyle about darting around the battlefield applying heals and dots as he goes and weaving in auto attacks to regain wisps. To help that, q and w would reset his auto attack timer.
E: This one's a new ability designed to let him choose when to apply the empowered effects of his q and w stacks, give him more ways to weave around the battlefield, and give him something he can use for waveclear. Cooldown would probably be around 14 seconds scaling down to 10.
Gideon dashes to and just past the target champion or large monster similarly to Rakan's e. When he arrives, nearby allies receive a stack of his q and nearby enemies receive a stack of his w. Removes any stacks present on the target when he arrived to instantly apply their remaining healing or damage increased by 10-50%. If the initial target had three q stacks then they are cleansed of crowd control, gain living armor which grants 25-35% damage reduction for 2.5 seconds, and releases a shockwave that slows nearby enemies by 40-60% for 1.5 seconds. If the initial target had 3 w stacks then a massive thornbush (aoe similar to lux e) erupts around them slowing nearby enemies by 30%, applying nearsight, and grounding them for 1.5-2.5 seconds. Can be self cast in which case Gideon turns invisible for 1.5 seconds instead of dashing.
R: 2 charges, 5-7 second duration each, and a cooldown of around 45 seconds scaling down to 35 per charge. Grants camouflage instead of invisibility with a detection radius of 400. He can attack and cast any abilities without breaking camouflage but taking damage increased the detection radius to 540 for 1.5 seconds.
This gives him a bit more flexibility about how he uses his stealth and let's him use his whole kit without wasting his ult. Camouflage instead of invisibility means the enemy can see him when he goes in to attack and it's less of a guaranteed escape. The overall duration is lower but being able to spread it out should help since between this and the potential invisibility on e he can use his stealth a lot more often.
