From the Bayrampasa Otogar in Istanbul, it's a 9 and a half hours trip. Some travelers use the modern city of Izmir less than 2 hours by bus as a base for exploring Bergama and other sites nearby Ephesus, Pamukkale and ÃeÅme, among others or the nearer town of Ayvalik approx. 45 minutes.
The main bus terminal is in the city outskirt about 7 kilometers away from the city center, so you can either take a taxi or the cheaper shuttle bus to get to the city centrum. But if you're coming from Istanbul and on a Metro Turizm bus, it goes to the city centrum passing by the main bus terminal.
The Metro Turizm buses from Canakkale leave you on the side of the highway, if you are dropped off on the highway carefully walk across to the Otogar you can see it visually across the highway. Be careful as it's a busy highway with lots of traffic. There are free Metro minibuses from the Otogar to Bergama city center. You may have to wait 30-40 mins though as their departure coincides with other bus arrivals. You can always take a taxi if you do not wish to wait.
The best and most convenient option from Eceabat or Cannakale is to catch a Soma Sehayat bus which goes directly to the Bergama city center so you don't have to deal with the above ordeals.
The Truva Bus Company runs busses from Canakkale which take five hours many stops but be aware - they drop you off on the highway from where you can walk to the Otogar you can see it on the other side of the highway or take a taxi which will no doubt arrive in the bus's wake - price negotiable YTL 10 was the best we heard about
