Prepending random data to encrypted file
It's much easier to add a cost factor to the key derivation from the password than to add a lot of data to an encrypted container. See the PBKDF's PBKDF2, bcrypt and scrypt for more information, or see this Q/A for GPG. In general, an iteration to derive the key is of course much more efficient than adding overhead at the start of the file.

If a key with an entropy over 100 bits and if there are no vulnerabilities in the encryption routine (this is likely true for GPG / CAST5) then brute forcing the key is already not possible, and the measly amount of time required to  decrypt a small amount of data is not much of an additional deterrent.

Note that CBC mode encryption can be decrypted from any location in the ciphertext, as long as the previous ciphertext block is available. As for the trick with the truecrypt volumes (wasn't that software deprecated?), how would you know as a legit user where to start reading/writing?