Here I am
Good to see more people enjoying this release!
I second Nacher's recommendations, and I would also suggest
Tiamat - Wildhoney: Released one year prior to The Gathering's Mandylion, and also sharing the same producer. Wildhoney has a very similar sound to Mandylion, crunching, yet beautiful guitars and great melodies, a sense of atmosphere and ambience. However the vocals are male and they are not excellent, but they suit the music somewhat.
Flowing Tears - Jade: Released in 2000, also sharing the same producer as Mandylion and Wildhoney, Jade provides us with the lush and midrange female vocals of Stephanie Duchêne, the music is not as atmospheric but the guitars are used in a similar way. The drumming is somewhat similar as well.
Chalice - Augmented: Australian band Chalice released Augmented in 2003, different from their past releases, including much more harmonic guitar riffing and wonderful solos, as well as Vocals of Shiralee improving significantly. I saw them live earlier this year and she gave me shivers! Good mix of more melodic songs with slower, doomier songs ... yet not even remotely sad, very arty sort of sound.
Artrosis - Posrod Kwiatow I Cieni: Polish Gothic Metal act, there is an English lyrical release called In the Flower's Shade however I wholely recommend the Polish lyric release as the vocals are much more fluid and natural, and Polish singing is melt in your mouth! Medeah is definitely a siren. Sonic signature similar to Mandylion I would say, although it is quite a bit faster.
Naamah - Resensement: Another Polish band, Progressive Metal this time however, english lyrics apart from the bonus tracks. Not exactly similar in sound and voice, although neither is Nightwish, however if you like Mandylion and the above releases, you will love this to bits. Great musicianship as well.
Brave - Searching for the Sun: USA Melodic Metal/Rock act, fantastic vocals of Michelle Loose, similar styles of playing to some of the above releases except with Piano. Highly recommended as well.
edit:
Also try
Lacuna Coil - In A Reverie
Moonlight - Inermis
Moon of Steel - Insignificant Details
Dreams of Sanity - Komodia
Leaves Eyes - Lovelorn