Ethereum uses asymmetric public/private key pairs to generate its accounts. So does GPG.
GPG key generation takes ages and needs lots of time/entropy. Ethereum doesn't. Why the big difference?
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By default, GPG generates a 2048-bit RSA key. This requires generating a very large random number that has structural requirements to make it a valid RSA private key. So not any random number will do. You might have to try several before you find one that meets the requirements.
GPG also typically creates both a master key and a subkey. So you're creating two keys, not just one.
By contrast, Ethereum's private keys are only 256 bits long and require no particular structure. Almost any random 256 bits will do, and the majority of the time you only need 256 random bits to generate one.
This is kind of an apples to oranges comparison though because GPG keys are suitable for both encryption and digital signatures while Ethereum keys are not intended for encryption.