Can I watch the sunrise inside Udawalawe National Park?

You cannot really be inside the park exactly at sunrise, but you can still catch the post-sunrise golden hour once the gates open at 6:00 AM.

Since the ticket-buying and entry process starts right at 6:00 AM, you will enter a little after the sun has already climbed over the horizon. Still, the early morning glow from about 6:15 AM until 7:30 AM is spectacular for photos, and it is also the span when wildlife is most active before the midday heat shows up.