The song "Sayonara" is sad by itself, but the live version for "Requiem et Reminiscence" is especially heartbreaking. And lets not forget that it seems like he has to stop halfway into Emu ~for my dear~ when singing live, makes sense since it's a tribute song for Kami.
"Kono Daremo Inai Heya de (in this empty room)". "Doooooshite...daremo inai kooono heya de...karada ga...furueru n darooo...oshiete." Na na na na na...
Which translates as "Why in this empty room, is my body shaking? Tell me..."
"Last Song" is quite saddening, especially because of the way Gackt sings it.
"Tsuki no Uta" is, too.
Other Tear Jerker songs by Gackt include "Hoshi no Suna" and the Gackt/Hyde duet "Orenji no Taiyou". The last one can cause some people to cry for all nine minutes and ten seconds upon hearing it. It doesn't help that it was used in a Tear Jerker movie (Moon Child). Equally tear-inducing is the live version, with all the characters from the movie except one very important one.