ATL has plenty of gates and it also has common use gates.
The airport itself does the job of making space available.
I'm not sure if WN has released its gates on D concourse but it doesn't have much if any flights there any more. Also, UA is consolidating its operations on the T concourse eventually.
The reason why ULCCs are expanding in ATL is because WN is decreasing its presence and because its fares have gone up quite a bit.
here is what F9 says
“Since Southwest bought AirTran, fares have risen in Atlanta quite significantly,” Shurz said. “We’re bringing back fares to the Atlanta market that customers haven’t seen in a number of years,” he said, including a one-day sale Thursday with fares from Atlanta starting at $19 to $99 each way
The latest DOT data shows that DL's share in ATL is unchanged while revenue is up.
DL still has a larger local share at ATL AND the largest local revenue compared to every large hub in the US except for EWR.
And it is noteworthy that DL's local revenue at its 3 NYC airports is higher than its local revenue at ATL.
so, DL does a very good job of maintaining its share in its hometown regardless of who shows up to compete but it takes a market the size of NYC to beat DL's own presence in ATL - and even there, DL holds the number one position at each of the two airports where DL has a hub.
and notably, NYC has plenty of low fare competition.
ATL and NYC work hand in hand in DL's network strategy and DL does very well in both markets and better than any other airline does in its top two cities.