How do I unpack or pack large quantities of gems at once?

There is no longer any need for Javascript hacks.

When you click on "unpack" or "pack" gems, Steam gives you a slider-bar to allow you to do as many as you want at once.

Visit your Steam inventory in your browser and select the gems or gem packs.

Hover over the pack or unpack button in your inventory and look at the link preview that appears at the bottom of your browser. Write down the number in single quotes after the 753.

Open your browser's console. In Firefox and Chrome it's CtrlShiftK. Enter one of the following:

To unpack:

setInterval(function(){ UnpackGameGooFromBarrel(753, 'REPLACE-THIS'); }, 1000);

To pack:

setInterval(function(){ PackGameIntoBarrel(753, 'REPLACE-THIS'); }, 1000);

Replace REPLACE-THIS with the number you wrote down (don't remove the single quotes). Then hit enter and wait while it unpacks each gem, one per second. If you want to speed it up, you may have success decreasing the 1000 milliseconds to a smaller number, but don't go below 250 milliseconds.

Warning: There's a small chance this may crash your browser after a while, so make sure you don't have any important unsaved work on a page before doing this. Decreasing the interval may make a crash more likely.