Yes, "It's gonna be a long day" is a correct phrase, often implying that the day will feel extended or challenging, either due to the amount of work or activities ahead or due to fatigue.
Here are some rephrased alternatives:
1. "Today is going to be a lengthy day."
2. "It looks like today will drag on."
3. "I have a feeling this day is going to feel long."
4. "This is going to be a tiring day."
5. "It seems like it will be a long day ahead."