Why is c25519 in the GPG “expert” options? The only way to access the c25519 curve in GPG is through gpg --expert --full-gen-key. Why is that? From my knowledge, Curve25519 is one of the most secure (and fast) elliptic curves in cryptography. So why is RSA fine, but Curve25519 is considered a risk only experts should take?
The risk mainly resides in compatibility.
See, not all GPG users/systems are updated to the latest version. If you look at the GPG changelogs, you'll notice ECC was first introduced to GPG with version 2.1 in 2015:


Support for Elliptic Curve Cryptography (ECC) is now available. ⇒more


None of the pre v2.1 versions of GPG support ECC, which is something that can and most probably will cause several annoyances and interoperability issues — hence the &quot;expert&quot; toggle which can be interpreted as &quot;herewith I confirm that I know what I'm doing and that I'm aware of related compatibility problems that may come within it&quot;.
Note that this is only an annoyance for the time being; things will surely brighten up while time goes by and more users/systems adopt newer GPG versions by updating
Also note the related dependencies and — last but not least — the OpenPGP specification &quot;draft&quot; status, which additionally influence adoption speed:

Encryption ECDH — RFC7748: Curve25519
OpenPGP specification: RFC4880bis draft
Dependencies:

libgcrypt &gt;= 1.7.0

Gnuk &gt;= 1.2.0



(Source: https://wiki.gnupg.org/ECC)