There are lots of ways to go to Gallipoli. It all depends of course on where you come from, but it is safe to say that not all means of transport are equally interesting if you want to visit the peninsula. So here is a satisfying way to do it.

To start with : most probably, you will go to Turkey by plane and once there, you will need a car. It's perfectly possible to travel to Gallipoli by public transport, but once you get there, you get stuck : civilisation ends about 10 miles from Anzac, 15 miles from Helles and 20 from Suvla.There is a sealed road, but nothing else.   
  
If you should decide to make the trip by coach anyway, the following information might be useful :

The first thing to do is to catch a bus at the Airport, that will take you to the 'Otogar', the big bus-station outside Istanbul, from where all long-distance coaches leave to destinations all over the country.

The price for the trip Istanbul - Eceabat is about 20 000 000 TL or something like 11 EUR / 15 USD.

Coaches are pretty luxurious and make a sanitary stop every 2 hours at a 'tesisler', a roadside restaurant, where you can get all kinds of refreshments. On board, smoking and use of mobile phones are forbidden.