If I buy a new terabyte external hard drive, is there any reason why I cannot use the Gnome disk application in Tails 0.17.2 to make the external drive into a LUKS encrypted volume?
I seem to recall there might be some firmware issues, and possibly also issues arising from a limited maximal size of a LUKS encrypted volume.
No, there isn't. It should work just fine.
LUKS doesn't depend on any specific firmware. As long as you can use the hard drive at all, you can also use LUKS on it.
And if there is a size limit, it is probably in the PB (petabyte) range, not TB (terabyte).