You can reach the Gallipoli city centre by land or by sea. You can reach Gallipoli from Istanbul via Tekirdağ and Keşan.
We’d like to point out that in order to go to Gallipoli from Çanakkale, you’ve to travel both by land and by sea. You can go from Çanakkale to Lapseki by land and then go from Lapseki to Gallipoli by sea or if you wish you can go from Çanakkale to Kilitbahir or to Eceabat by ferries and continue the rest of your journey by land.