"It was a celebration of the capture and execution of anti-government forces, with some vaguely anti-Catholic undertones"
The Gunpowder Plot was a plan to blow up the (Protestant) King and politicians and replace with Catholic alternatives, including the possibility of replacing the current Protestant line with a Catholic monarch and more pro-Catholic politicians. Hence the celebrations centred around the failure of a Plot to kill the monarch, and celebrate his continued good health. Given the political and religious context of the times, this included strong, rather than 'vague' anti-Catholic undertones: for many years it was traditional to burn an effigy of the pope, and the famous folk verse includes the lines:
" A rope, a rope, to hang the Pope,
A penn'orth of cheese to choke him,
A pint of beer to wash it down,
And a jolly good fire to burn him. "
Less revolutionary Catholics of the time feared a revolutionary responses to the suppression of Catholics in England at the time in case there was a backlash from the authorities, and indeed this did happen with the discovery and failure of the plot.
Probably both sides were as illiberal as each other at the time, lots of nasty things were done in the name of God and the King across Europe.
I'd very much agree with you that people have re-interpreted the event to their own ends. I don't think the plotters were in the least bit bothered about votes for women, abolition of slavery, replacing a monarch with an elected president, and would happily have burnt anarchists at the stake if they'd found some.