Everyone Is Bi in The Wicked Years series until proven otherwise, so this is a given.
Elphaba and Glinda have a considerable amount of Ho Yay in both the musical and books. Their duet at the end of the musical is arguably more romantic than any other duet in it.
Hell, their first duet is probably the walking definition of Slap-Slap-Kiss romance ("My pulse is rushing / My head is reeling / My face is flushing / What is this feeling?" No Elphie, it's not loathing — it's lust.) Word Of God is they tried to find every sappy romance lyric, put it into the song, and subvert it; so its very much intentional.
It doesn't help that the first time we hear the Wicked 'Unrequited Love' tune, it's when Glinda tells Elphie she's beautiful. Elphie's weirded out and runs off. Is it just me or is Glinda a teensy bit wistful as she sings about E protesting her disinterest?
And it gets even more blatant during the Emerald City sequence ("One Short Day / Defying Gravity"). Two... best... friends, sharing one wonderful day. Riiight. By the time Elphie sings "Kiss me goodbye, I'm defying gravity," the Yay is about as thick as poppy fumes.
This is subtler but definitely intentional in the original novel; by the end, Maguire's done all but come out and say that Glinda's been in love with Elphie ever since their schooldays. (Whether Elphie ever felt anything reciprocal is extremely open for debate, since iirc none of their scenes together are from her POV.)
Glinda never had a romance with Fiyero in the books, and it's explicitly stated she married her husband for money and that she never had sex with him. When you put this together with her relationship with Elphaba, it comes that she's supposed to be gay.
Word Of God has said ""something" was going on between them in a romantic sense, but he never clarified on it after that. Many of the musical cast (including the original Elphaba, Glinda, and Fiyero) have said that the musical is a love story between Elphaba and Glinda and that they would have dated each other if things had turned out a little differently.
On page 178 of the first book in the series, Elphaba and Glinda share goodbye kisses (two in fact). While arguable that this is just because the book is set in a time where girls were more open toward each other, the "Her head had turned away quickly, not to hide her tears but to soften the fact of their absence. But the sting, to Glinda, was real. " line makes it rather clear. Less than ten pages beforehand, we have a scene where Glinda and Elphaba have a Sleep Cute scene.
The Les Yay extends to the sequel book, where Glinda still seems very upset over Elphaba's death. When Liir, Elphaba's son, brings her Elphaba's cape she refers to her as "My Elphie".
In "Thank Goodness", Glinda sings wistfully about how she feels like she ought to be thrilled to be marrying Fiyero, but she can't help but think of "a couple of things that got lost/and there are bridges you cross you didn't know you crossed until you've crossed...".
"For Good" sounds so much like a love song, especially the description of nature that was compared to the fact that Glinda and Elphaba knew each other and changed for the better.
Liir himself is in love with both a man and a woman.