I'm a PC in a D&D 5e campaign being run by a friend. During an encounter with some mephits, I used Gust of Wind to try and keep them from swamping the group as we have no one in a tank role. The DM claimed that the pushback effect is only active on the first turn of the spell's activation. As I read the spell description, I would think if that were true there would be a qualifier in the text, but I don't see one.
Gust of Wind
2nd-level evocation
Casting Time: 1 action
Range: Self (60-foot line)
Components: V, S, M (a legume seed)
Duration: Concentration, up to 1 minuteA line of strong wind 60 feet long and 10 feet wide blasts from you in a direction you choose for the spell’s duration. Each creature that starts its turn in the line must succeed on a Strength saving throw or be pushed 15 feet away from you in a direction following the line.
Any creature in the line must spend 2 feet of movement for every 1 foot it moves when moving closer to you.
The gust disperses gas or vapor, and it extinguishes candles, torches, and similar unprotected flames in the area. It causes protected flames, such as those of lanterns, to dance wildly and has a 50 percent chance to extinguish them.
As a bonus action on each of your turns before the spell ends, you can change the direction in which the line blasts from you.
I don't see a qualifier that constrains the pushback effect to just the first turn, and I feel the spell's usefulness is severely affected by this condition. Especially for a level 2 spell slot.
Am I wrong about the spells effect? If I'm not, how best could I convince the DM not to nerf my spell? I'm a Circle of Land druid and feeling very weak in a direct damage sort of way. I was close to feeling useful to the group when I pulled this trick out and I feel like I was undermined arbitrarily.
Best Answer
The pushback lasts 1 minute or until your concentration is broken
The rules
When it says that the wind is blowing for the spell's duration it means that the wind is blowing for 1 minute (or until you stop concentrating on it).
The next line says:
So the line of wind is present for up to one minute, and, as long as that line is present, creatures that start their turn in it must save against its effect or be pushed back. The applies to every turn that a creature starts its turn in it for the record.
I'm afraid your DM is not following the rules as written. I think it is pretty clear that the spell works, and is intended to work, as I have described above.
Common sense
The fact that this is a concentration spell should be a giveaway that some continuing effect should be present. If not, what reason would there be to concentrate on it? What lasting benefit does it provide if not the titular wind?
Does it makes sense to have "strong wind" constantly blowing against people and not have them make saves or have any effect at all? It really doesn't.
Talking to your DM
Whatever you do, you must respect the DM's right and ability to modify the campaign as they see fit. However, you should bring up to them (privately if at all possible) and respectfully ask them to look it over again and see if they might have misread or misunderstood the spell.
If they are still insistent in the previous interpretation, explain to them how badly this nerfs the spell (especially compared to other 2nd level spells). Ask them if they would mind interpreting it your way for a session or two and see if they think it is overpowered (because it is not) or anything wrong becomes apparent to them.
If none of the above works, you may just have to live with it and might consider just not preparing it again if you think it is not worth it. You may also want to ask the DM if you should expect any other changes to rules/spells/abilities to head off any future misunderstandings (thanks @MC).
If there are other serious issues that are seriously affecting your enjoyment of the game you may want to consider starting to look for another group. However, do remember that this is only one spell and probably not worth ruining an otherwise stellar gaming experience for.