Just a note on metaphor: a gallows is also known as a "stairway to heaven."
The meaning isn't terribly deep, but thematically the song is about the uselessness and shallowness of possessions:
"There's a lady who's sure all that glitters is gold / and she's buying a stairway to heaven."
The line "all that glitters is gold" is actually an allusion to the line in Shakespeare's The Merchant of Venice. In Act 2.7, the Prince of Morroco has been tricked. Expecting to find a picture of Portia in a golden casket, instead he discovers a picture of Death with a message written in its empty eye socket:
"All that glisters is not gold."
A woman tries to buy her way up the stairway to heaven. In the beginning of the song, Robert Plant sings of a "lady" who apparently is wealthy as she has "hedgerows" and other things. She seems to be attempting to buy her way to heaven expecting her entry to be the same as all her worldly entrances and purchases.
In the 1960s there was much rebellion against the Establishment, both in America and in LED Zeplin's England. This song seems to be on the subject of the hypocrisy of this Establishment. It is difficult, if not impossible, to decipher this song because the group stayed up all night, composing it at a place in the country; the likelihood that their creative venture involved drugs is extremely high, of course. So, some lines are a bit incoherent.
According to interviews with Robert Plant and Jimmy Page there was no specific meaning to this song. It was the most requested song ever played on mainstream radio. It has been played for weddings and funerals and everything in between. One of many questioned lyrics was, "If there's a bussel in your hedgerow.." This is simply talking about the beginning of spring. I have hedge in my back yard and when spring arrives there are so many birds and animals hanging out in the hedge row it looks like it is alive. Every bird is busseling around finding space for nests, food, and a place to play. In an interview Plant stated:
"It (the song)was done very quickly. It took a little working out, but it was a fluid, unnaturally easy track. It was almost as if—uh oh—it just had to be gotten out at the time. There was something pushing it saying, 'You guys are okay, but if you want to do something timeless, here's a wedding song for you.'"
"Robert Plant once stated that the song was "...a women's quest for spiritual perfection" and ".. a simple wedding song" Jimmy Page said, more sincerely, that the song has whatever meaning the listener wants to put to it. It was a song of hope."
The song is a slap at the "church". The lavish churches, indulgences(buying time out of purgatory) and the silly notion that having the May Queen pass you dissolved all your sins (bustle in your hedgerow). The song points out the different oddities of the church vs. true religion.
And in the end the church will continue to buy the stairway to heaven.