Sage Advice gives us a guidance on how to know if a spell can be dispelled or not:

Can you use dispel magic on the creations of a spell like animate dead or affect those creations with antimagic field?

Whenever you wonder whether a spell’s effects can be dispelled or suspended, you need to answer one question: is the spell’s duration instantaneous? If the answer is yes, there is nothing to dispel or suspend.

Wish does have instantaneous duration, so it is not questionable that its effects cannot be dispelled, except one:

You grant up to ten creatures you can see immunity to a single spell or other magical effect for 8 hours.

Strangely, this is the only effect offered by wish that includes duration.

Does this mean that if I use wish to gain this effect, it can be dispelled?

No. The duration of Wish is instantaneous

In the quoted Sage Advice, "duration" refers to the the "Duration" field of the spell's description, not whether the spell has a lasting effect. Wish has a duration of Instantaneous; therefore, the spell is instantaneous, even though it has a lasting effect.

I realize the example you give involves an ongoing effect created by magic. When someone tries to dispel it an hour later, though, the catch is that the effect was created by magic. After it was created, the effect simply existed, without the aid of ongoing, dispellable magic.

As the Player's Handbook puts it:

Duration

A spell's duration is the length of time the spell persists...

Instantaneous

Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can't
be dispelled, because its magic exists only for an instant (PHB p.
203).

Compare this to Plant Growth. It can either cause plants to become overgrown or cause soil to become enriched for one year (depending on how you cast it). The spell's duration, though, is Instantaneous. It cannot be dispelled.

• The overgrown plants are simply overgrown. If you want them gone, get
  a lawnmower.


• The enriched soil is simply enriched. If you want it
  un-enriched, wait a year or spray the field with herbicide.

Also compare to Fireball, which can burn people and set things on fire. The spell is instantaneous; it cannot be dispelled.

• If it sets your sofa on fire, your sofa is now covered in nonmagical fire; use a fire extinguisher.


• If it injures your fighter, the burns are real, but there's no magical residue; use Cure Wounds.

This is unclear; ask your DM

As you noted, wish has an instantaneous duration. There are two ways to interpret this effect in that light:

1. This is an instantaneous effect that lasts for 8 hours - Not dispellable

This is a weird one, but it could theoretically be that the spell and its magic are instantaneous but that the protection it offers lasts for 8 hours. This would be an odd interpretation to say the least given that the vast majority of spells' effects follow their duration (see plant growth discussed in the last section).

If you were to side with this reading, it would mean that you could not dispel the effect since the spell that triggered it was instantaneous and there is no longer anything to dispel.

2. The effect extends the duration of the spell - Dispellable

In this reading, this specific effect has its own duration separate from the duration of the spell and could be seen to override that duration. Again, I don't know of any spells that do this so it is a weird edge case. However, if you read it this way it means you would have the effects of a 9th level wish on you for 8 hours and that a 9th level dispel magic could remove it.

Neither is obviously better

Honestly, neither option seems significantly more compelling than the other mechanically. For what it is worth, plant growth also has wording similar to this, but it also has the exact same ambiguities in that the wording can be interpreted in either of the two above ways.

Out of them, #2 is a bit more intuitive though to me in that it just extends the duration of the instantaneous magical effect into an 8-hour-long magical effect. On the other hand, #1 adheres most strongly to the spell as written (following the duration as listed).

Being that this is ambiguous, your DM will have to decide which is the best option for the table.

Yes, it is a spell effect and can be dispelled

Many spells can create magical effects that last beyond the stated duration. For example, teleportation circle has a duration of 1 round, but in the spell text, it says:

You can create a permanent teleportation circle by casting this spell in the same location every day for one year.

Just because the duration of the spell is listed as 1 round doesn't mean that the permanent teleportation circle cannot be dispelled after 1 round has elapsed since the final casting. In fact, there are multiple questions centered around the dispellability of a permanent teleportation circle.

Similarly, even though the listed duration of wish is instantaneous, it is still capable of producing magical effects that last longer than that. (In fact, it does so every time you use it to duplicate another spell with a non-instantaneous duration.) So, if you believe that a permanent teleportation circle can be dispelled, then you must conclude the same for any ongoing effect from wish.