Yes, and no.
You can connect Easel to the internet and use it on a project without the internet after the initial log-in. You can also leave Easel open and run your machine with new files, but they won’t be saved unless you re-connect to the internet. Some people use their cellphone connection to log-in and get a project started and then turn off the network while they are working and running the machine and then log back in later and let their file save up to the cloud.
Like @NAM37 I would recommend using Universal G-code Sender as it is fully offline after it is installed. (use nightly builds version 2)
ChiliPeppr is a great option, but it is online as well.
Edit: I have left my browser up with Easel up and running for a few days at a time before.