Why is my Bitcoin private key so much shorter than my PGP private key? My PGP key takes up a huge, multi-line block of text, but my Bitcoin private key is just a small string. Yet both are considered extremely secure. Why is my PGP private key so long?
GPG keys are usually RSA keys.
RSA keys are based on the difficulty of the factorization. Keys are usually 2048 or 4098 bits.

Bitcoin uses ECDSA (Elliptic Curve Digital Signature Algorithm), where keys are usually smaller. Bitcoin uses the curve secp256k1 where keys are 256bits.

Roughly, an RSA key with 2048 bits offer similar security then a 256 bit ECDSA key.
Both offer more or less 128bit of "symmetric" security.

With newer versions of GPG you can also generate and use EC keys (nist-P, ed25519 or secp256k1)