PARI/GP first release was in 1986 (before version 1 of the GNU GPL).
The GIT repository start a bit before version 2.0.17 (1999/09/16).
The first release under the GNU GPL was 2.1.0 (2000/11/17).
To cut any speculation: the MPC developers were well-aware of PARI/GP when they decided to write MPC (and conversely, the PARI/GP developers were well-aware of MPC development).