I didn't spell it out because I thought it was a given, but my mistake - the way the aura counts as difficult terrain means that someone charging the Cleric will (probably) not reach him in time to do an attack.
This means the attacker has to endure the damage twice: in the first round when entering the aura, in the second from starting in it. Before even getting the chance to make an attack, I mean. Obviously it's better if this damage doesn't reduce you to tatters.
Great, then we're on the same page with respect to the rules* and the cleric's tactics, because this is indeed what I initially assumed, since it's the standard way to use Spirit Guardians--but it also means that gnoll tactics are the major variable, as per the math I outlined. Gnolls who start out more than 25'+ away from the cleric and then charge into the aura, and then Dash again to get themselves within 5' of the cleric, will have an 80% chance to die before getting any attacks. Gnolls who are willing to do
that after seeing 80% of the dozen or more gnolls before them get shredded doing the exact same thing are clearly some kind of berserker. (In fact, they're likely clambering over the corpses of the previous gnolls.)
If that's the case, then this isn't even about 5E or Spirit Guardians: those gnolls would apparently run through fire or a Wall of Fire or an army of zombies or off a cliff to attack the PCs, in AD&D
or 5E. How surprised am I that they died gruesomely and ineffectually at the PCs' hands? Not at all.
* Technically it's not difficult terrain--instead it halves your speed, which is cumulative with difficult terrain, which makes it even better.
This means Spirit Guardians make you practically immune to melee attacks from anyone that doesn't have the Hit Dice. This is why I want variants of humanoids with CRs up to at least 5, and why I'm disappointed by the MMs strong focus on CR 1/2 and CR 1 humanoids.
Even if they do have more Hit Dice, that just means you need to stack a Wall of Fire
and Spirit Guardians to do 9d8 (38.25 after saves) instead of 4d8-save-for-half (15). The root problem is their lemming-like desire to attack with their bare hands through any obstacle.
The moving forward bit is to inflict damage on anyone trying to stay at the edge of the aura. In practice it extends the danger zone by at least ten feat (assuming you will want to return to your original position at the end of your turn).
Anyone you damage this way is almost guaranteed to get at least one attack (82% chance per earlier discussion--there's a 23% chance of rolling at least 22 damage, times 0.75 chance of a failed save which applies the full 22 damage, equals 17.5% chance of killing the gnoll before it attacks), which means that even against an AC 20 target the 70 gnolls can expect 14 hits per round, about 70 HP of damage.
Gnolls who end their turns outside the aura
don't have to endure the damage twice before attacking, and if they have any sense at all they won't. If they don't have any sense and will charge through an army of faerie guardians just to get to the PCs, then they can be killed any number of ways--the fact that the cleric used Spirit Guardians instead of Silent Image and a pit trap is practically just a detail. ("Practically" because I'm not a mind reader, and maybe you have reasons as a DM to make them react differently to Spirit Guardians than a pit trap or Wall of Fire--you did say that you had it be just a "slight shimmer". Was that because the gnolls would have paused in their headlong rush if they'd been able to see what was slowing and killing them? But based purely on what you've said I have to assume that they would have charged through any obstruction.)
I am not saying that was a bad encounter or that you are a bad DM.
Edit: oh, wait, when you say "assuming you will want to return to your original position at the end of your turn," are you talking about the
cleric moving forward and then back while Dodging? When I first read your post I had assumed that the cleric moved forward to make additional gnolls start within his aura, because that
is the standard Spirit Guardians tactic. But there's no point in moving backwards again, since that just takes those gnolls right back out of the aura again before they have a chance to take damage. If the cleric does that, nothing happens. No gnolls enter the aura during his turn (in the 5E jargon sense--the aura moves over them, but they do not "enter" it), no gnolls take damage, and he's back in his original position when they start their turns, so exactly the same gnolls take damage as if he hadn't moved.