Does anyone know if and how I can play Dragon's Lair on my Retropie?

It is possible, but given the matter of obtaining the er, needed software isn't really something that I think GOG's guidelines are conducive of.

Are any games on GOG available for Raspberry Pi?

If it runs in Dosbox, yes. Easily.
If it doesn't run in DOSbox, that's an asterisk.

Edit: Also I completely forgot Dragon's Lair was ported, my bad.

PI is not a supported OS, so you won’t get support. Nothing stopping you trying though.

The Dragon's Lair sold here is a Windows port. It won't run on a RPi.

If you are talking about the GOG version, it is meant for x86 CPU PCs running Windows.

Raspberry Pi has a different CPU architecture (ARM) and the OS is a Linux variant, so no go.

But if any port of Dragon's Lair is fine with you, then I guess you could try to run the Amiga version of Dragon's Lair on Retropie, as it apparently supports Amiga emulation as well. The Amiga (floppy) version is a pretty poor port though, but back when it appeared it was a technical marvel (how well it mimicked the original arcade version).

Or maybe the SNES version, as I recall there was a SNES port of Dragon's Lair as well?

EDIT: Oh, somehow it didn't occur that the original arcade version could be emulated on Retropie as well:

https://www.youtube.com/watch?v=yd0QAfYllII