Is it a bad idea to use PGP encryption for structured/predictable data? I'm working on an app and need to encrypt some structured data. The data will be turned into a string form (JSON) before getting encrypted.

The data format I'm using will always start with a { character and end with a } character. Does this make encryption less secure? Are there any methods for more securely encrypting a JSON string?
There are no known-plain-text-attacks known for the symmetric encryption protocols used by OpenPGP with the OpenPGP CFB mode in use, so you can safely rely on it.

In the end, the very first few bytes of OpenPGP packets are generally rather easily guessable by design, as they usually introduce a compression-packet and literal (data) packet with either very small parameter ranges each -- and this is not considered a problem for OpenPGP but for some very esoteric and hardly usable adaptive cypertext attack relying on a decryption oracle, that is partially exploiting this.