I think you don't need to run a full node just to transfer them to your ledger nano.
Electrum is enough to transfer and sweep them in your Ledger nano but your problem is you can't able to verify the signature?
If the gpgsuite doesn't work in Mojave you can try other software to verify the signature.
Try this GnuPG
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https://bitcointalk.org/index.php?topic=5105901.msg49695453#msg49695453Sir if you could please help...
I have Gnupg installed but in the tutorial you posted it references the Linux version (.ter.gz) of Electrum and it's corresponding key.
When unpacked, this package has no use on OSX that I can tell.
Is there any way you can direct me to how to verify the electrum-4.1.2.dmg file and it's corresponding signature file with Gnupg from the command line please?