Yes, you can use both "will" and "be going to" after "from now on," but they convey slightly different nuances, as you mentioned: "From now on, I'll work out on a daily basis" suggests a decision made at the moment of speaking, indicating your intention to start a new habit right now or in the very near future. "From now on, I'm going to work out every day" implies a decision made in the past or a pre-existing plan to start this new habit. It conveys a sense of determination or commitment that was established before the moment of speaking.

They have the same meaning but they feel different. "Will" is a modal verb, and like the other modals it expresses levels of certainty or possibility. Of all the modals, "will" is the only one that expresses maximum certainty. That is its primary purpose, not the creation of some future tense (although it can do that too since things that are certain to happen *will* happen). "Going to _" is an adjective phrase. It's descriptive. It has no force. It's just a picture of whats "gonna" happen. If you really want to express your determination to work out on a daily basis, use "will".

You can use either one. I prefer the "going to" future in your example, but using "will" wouldn't sound strange or unusual in any way.