I don't remember the bus schedule but we used it to get to the bottom of the steep hill leading to the Canopy Tower. If you feel like you'lll need a cab to get to the Rainforest Discovery Center, then you'll surely need one to get to the top of the hill that the Canopy Tower is on. When starting out on Plantation Trail, we were accosted by a ranger who asked for an entrance fee for the day. We saw him again while walking Pipeline Road. We said we were walking to the resort and he didn't charge us then. Apparently, the fee for the PRDC covers the daily fee for the park. Speaking of entrance fees, after walking up that very steep hill to the Canopy Tower, we were told we could not bird the Canopy Tower or grounds unless we were staying there. At $110 per night EACH, we declined. They had openings that night but would rather the rooms be empty than give a reduced price for walk up traffic.
As I mentioned, I don't remember seeing taxi's in Gamboa. Possibly, that type of transportation is more of a side business for one or more of the locals since the town is so walkable and the buses are frequent enough to go just about anywhere else. You might have to ask around for someone who would provide that service. The Resort might be of some help since they probably service clients who can afford to hire taxi's.
True, if you want to get to the PRDC (Panama Rainforest Discovery Center) at dawn, you'll have to get up early and walk in the dark. We did that once and spent the first morning hours listening to the dawn chorus near the entrance to Pipeline Road. Very few of the sounds were birds we could identify but the experience was exciting. As I recall, there were very few bothersome bugs.
Two of the photos attached may show you how walkable the town is. There are nicely built and well kept homes, paved sidewalks, landscaping, maybe a bit more upscale than what initially comes to mind for a small town in Central America. This little store is about the best option you'll find though for convenience food supplies. You might want to stock up before arriving in Gamboa! Finally, two lesser known trailheads that are right in town are pictured. You may have to ask about finding the trailhead to the watertower. There was a lek of Red-capped Manakins near the top and a continuation of that trail going past the tower that we never explored. Be prepared for a leg workout however; it is a steep climb. The fairly short La Laguna trail is near the Resort and may well be on the Resort property. It was not particularly well kept when we were there but the birdlife in the second growth habitat was terrific and it was less than ten minutes walk from where we were staying.
I hope all this helps; I did post a trip report for our visit back in 2010.
Steve