From Lembar either drive ride yourself to Tembowong (the boat landing for the island) or catch a taxi from Lembar (200,000-250,000 rupiah) or bemo (30,000 rupiah). If you're aiming for a bemo, you'll need to hike out of the port area for about 1km to the main road as they are not allowed into the immediate port area. The trip to Tembowong should take 1-2 hours and is a scenic trip during daylight hours.
Gili Gede is reached by a short boat trip from Tembowong where a boat should cost 25,000 rupiah per person for the run over. The trip time is 10 to 20 minutes or so, depending on where you are going exactly. If you have a reservation, the resort may arrange to have a boat waiting for you, but in practice this is not necessary.
The "port" is beside the petrol station that sits at the eastern end of Tembowong. The whole joint is tiny -- blink and you'll miss it. If you stay at Bola Bola or Palm Garden, ask staff to arrange you a boat from there to the island -- there is no need to head back into "town" to get a boat to Gili Gede.