A very common material for gaskets, before around 1840, was oakum, picked-apart old rope. As rope had a limited lifespan before it became "long-jawed" and lost a good bit of strength, there was a good supply of cheap old rope. It was sometimes picked apart in workhouses, and was tarred and hammered - caulked- into seams in carvel-planked boat hulls, or was greased and stuffed around the edges of steam pistons on Newcommen-style engines, then held down with weights. It could also be mixed with pine rosin and shellac ( gasket shellac, which is still sold) If you get valve packing from a hardware store today, it's essentially the same stuff that was used for the same purpose in 1800, though perhaps now with graphite instead of grease.

Leather gaskets were also common , for things like water pumps. But leather tends to eventually break down when immersed for long, and also comes apart when hit with steam. Still, you can still find leather flappers for those classic lever water lift pumps.

In the 19th century steam engine mechanics experimented with various materials. As steam pressures rose, things like oakum worked less well. One method of sealing two nesting steam pipes circa 1820 was to pack iron filings and sulphur powder into the gap and apply water, which created a very solid iron sulphate seal... which was likely also hard to remove.

There wouldn't be useful rubber gaskets until Goodyear ( or Hancock) discovered vulcanization, so not before about 1850. Before then, rubber ( or guta percha) was quite sensitive to heat.