How reliable/useful are pogomoves.com Gym Offense Ranking?

I don't like that site, I think they account only for DPS, which can be usefull just in 1 on 1 situations and consider short battle as supreme metric.

Better alternatives:
1. This own site :): https://pokemongo.gamepress.gg/gym-attackers-tier-list - read the commets for each mon, very helpfull
https://pokemongo.gamepress.gg/gym-defenders-tier-list
2. pokebattler varios tops you can create on your own:
https://www.pokebattler.com/rankings/attackers/levels/40/defenders/levels/40/strategies/DODGE_ALL/DEFENSE?view=SINGLE
or for more details see those from pokedex, here is for blissey: https://www.pokebattler.com/rankings/attackers/levels/40/defenders/levels/40/strategies/DODGE_ALL/DEFENSE?view=SINGLE (see the Best Counters area)
3. goptimiz.fr/en - here is Caounters for Blissey: http://goptimiz.fr/en/pokemons/blissey#counters

Pakebattler let's you customize your top most, you can see the top based on power, time to win or Overall, which is a combination of the two above (don't know with what percentages).

Tl;DR Gamepress tier lists and Pokebattler.com. Play with pokebattler, offers a lot, once more, a lot, of valuable info (to the details like what level should be your mon so that he deals max damage to a high level defender - i.e. Machamp has to be lvl 32 for Counter to deal 10 vs. a lvl 35 Blissey, otherwise will deal at most 9)

These are raw DPS numbers. If you want the absolute quickest way to mow down a defender and have an abundance of potions, their list is the most accurate one.

To your queries:

1.
Vaporeon is very weak for raw DPS numbers. It is a tank, but is slow on the attack. Attack stat of 205 is on the lower side of top pokemon. Water gun/hydro pump is a good moveset but it is not nearly as damaging as some others like Fire spin/overheat flareon (not to mention the monster attack stat the flareon has). Not saying Flareon>Vaporeon overall, but Flareon (and many others)>Vaporeon for attacking speed definitely.

2.
Dragonite is the better counter against dragonite anyway, lapras doesn't even come close to dishing out enough DPS to make the top list. Cloyster, Pilswine, Jynx etc. are all stronger than lapras.

3. Gengar is the highest DPS attacker in the game (but is very fragile!!). I'm not surprised if it performs well against a neutral matchup (vaporeon), or does the best in an super effective matchup (exeggutor). Gengar does get taken out in 15 seconds though (unlike dragonite), but it also inflicts heavy damage to the opponent.
4. A solar beam fired from an attacking exeggutor can nuke almost any pokemon. With lapras being weak to it, I'm not surprised at all.

That said, I use that list because I have an abundance of potions and gym turnovers are high (almost never goes beyond level 4 here). Hence I can solve everything quickly with my trio of gengars + 2 dragonites in reserve. I also pick up way too many potions to use.

If you are in a low pokestop density place with stable gym scenes, I'd not recommend using that site as a definitive guide.

Looked at Alk and Gary vs Blissey.

The fact that Alk is #9, and Gary is #103 tells me the list is garbage.

When you play with this combo on Pokebattler.
Gary is actually FASTER than Alk (who can't even win vs 1 of the 6 blissey movesets)

The issue being they don't account for Damage Recieved based energy in thier list. So pokemon who don't have to dodge Quick Attacks and can soak them and use that energy, are actually higher dps than pokemon who do have to dodge quick attacks.

(Part of my ever present Gary > Alk campaign for this site!)

It is reliable data.

On top of “Gym Offense” you should look at the ranking “Dueling Ability”.

Gym offense ranking: which Pokemon will damage the fastest? (If you want to win fast and do not care about the number of revive needed).

Dueling ability ranking: which Pokemon will do the most damage before fainting? (If you want to be revive efficient and do not care about the time it takes you to destroy a gym). This corresponds more to the GP approach.

I personally use a combination of both lists to identify the best pokemon for a situation. For example, the best Blissey slayer is Dragonite in my opinion. It is only 8th for dueling ability and only 5th for gym offense, but on average of both lists he is first.

1. Against Snorlax and Blissey, Vaporeon with WG/HP is ranked 182nd?
   Indeed, Vaporeon is quite slow. it is number 17 for dueling ability.
2. Against Dragonite, Lapras with FB/Blizz is ranked 39th while 5 Dragonite movesets make the top 10?
   Dragonite is clearly the best and in particular the fastest. Lapras is ranked 7 (only after blissey and the best dragonite) for dueling ability.
3. Gengar with SC/SB is ranked 3rd against Vaporeon and 1st against Exeggutor?
   Gengar is the fastest but fall at position 134 for dueling ability.
4. Against Lapras, Omastar with RT/RS is ranked 32nd and Poliwrath with RS/DP is ranked 122nd, while Exeggutor with Conf/Solar is ranked 2nd and Dragonite with DT/OR is ranked 10th?
   Omaster and Poliwrath are very slow.

Your post reminded me of world of Warcraft. There were some geared guys with high lvl items which "can" do more DPS on paper but will they last long enough to make it worth?Defender will attack back and you have to count that too. Strategies my man strategies...

pogomoves is accurate in term of dps and if you don't consider dodging (e.g some moves like dragon breath, water gun, bite, etc are very good for dodging and it affects survivability), damage received, and type-disadvantage received by the attacking mon. An example is one of the points you mentioned: dragonite is better than lapras when attacking dragonite. This makes sense if you only consider dps since dragonite has way higher atk stats than lapras. But they didn't consider the tankiness and the fact that the defending dragonite might have dragon-type moves e.g. dragon tail + outrage which also deals super-eff damage against attacking dragonite. I would say they lack deeper analysis unlike gym attackers tier list by gamepress or gym attackers tier list by ryanoftheday from thesilphroad on reddit. But you can refer to pogomoves if you are looking for the best movesets of a certain pokemon species in terms of dps.