Windmill Lay gauge stuck

Does the sidelay move freely, independent of the spindle running through the platen?

Are there any signs of collision between the sidelay cam and the gripper driveshaft?

Both can be checked with the press closed.

What precisely got stuck in there ,did you have a lay in the wrong place ?
Is the long bar that goes through the platen body free moving .?
Have you bent the drive link (a long flat bar) .
If you are lucky it may only be the bronze bush that goes through th platen is binding , if the steel drive shaft flexes in a jam sometimes its” memory” saves it from bending permanently but the long bush it passes through has no memory and binds on the shaft because it has bent.
It may also be as simple as just loosening the two bolts that bracket the drive bar to the rear of the platen and re shimming it till its free ,this may require the removal of the pins or at least one of them as the pins are there to stop movement in the bracket and when reshimmed will need reaming out and new pins replacing once everything is retightened .

Indeed, shimming with paper was enough to free-up the gauges on a machine I resurrected last year.

In that case, lack of lubrication was the likely cause of the bar crashing into the driveshaft.

http://www.briarpress.org/35274

the long bar isn’t freely moving but doesn’t look bent, although think that’s the offending part.

The flat lay bar is definitely bent but now that’s off the workings still don’t move freely.

An imperceptible bend can cause the spindle to bind.

The link bar and arm (near the flywheel) that drives the spindle can be disconnected by undoing a bolt, to isolate the spindle. If it still binds, the bracket on the back of the platen can be loosened to see if the spindle frees.

You wont likely find the long bar bent as there is not sufficient play to overcome the memory but the bronze bush may be bent as mentioned above loosen the bracket on the rear left end of it where it bolts to the platen rearside.
remember it has holding pins so remove the bolts first and a nylon mallet gives enough shock to loosen the block you may if lucky only have to deal with this area ,information reference this block through which the lays drive shaft runs is lacking in the manuals ,its position is obvious once you look closely .