Generally speaking: if your RSA decryption implementation has any timing sidechannels, then it is probably vulnerable to some kind of Bleichenbacher variant.
As far as I know, GMP doesn't many any attempts to be generally resistant to timing channels. This year's Bleichenbacher variant[1] specifically calls out GMP's modular exponentiation API (which is what you'd use for RSA) as being susceptible to timing.