Just done one on My GT.
Here's what worked for me.
Soaked the post/tube in WD-40 overnight.
Bight pair of Stilsons on the post and get someone else to hold frame.
Hard work, but got it un-seized. Post was junk.
Inside of seat tube cleaned/de-rusted.New post inserted with lots of copper-slip. Should be OK for next 15 years (assumimg I don't grow/shrink)