Gambella can be accessed by road and air from highland Ethiopia. By air it only takes 55 minutes from Addis Ababa and two days by bus from Addis Ababa as well. If you choose to come to Gambella through wellega that is Gimbi Nekemte route it could take you two days also. However, that is not a designated and a popular route. Your best chances of reaching Gambella within two days can be minimal as you might struggle a little bit finding a public transport in Dembidolo to Gambella.
Therefore unless you want to tour western Ethiopia through Ambo, Nekemte and Gimbi route and possibly spend some more hours or days in Dembidolo if you don’t find a vehicle to Gambella, your best option would be to just travel through Jimma Metu route popularly used by many passengers.
If you travel from South Sudan to Gambella you can get through it by road on many weredas bordering Gambella with South Sudan or by river through Jokow wereda if that is a winter season. These include Lare, Wanthoa, Akobo, Jor and Dimma. A trip to Gambella via all these weredas may only take half a day. However, it could be a full day or two via Dimma which links with Gambella through SNNPR and Oromia regions.