This version of GMP is upwardly binary compatible with all 5.x, 4.x, and 3.x versions, and upwardly compatible at the source level with all 2.x versions, with the following exceptions.
mpn_gcdhad its source arguments swapped as of GMP 3.0, for consistency with other
mpf_get_preccounted precision slightly differently in GMP 3.0 and 3.0.1, but in 3.1 reverted to the 2.x style.
mpn_bdivmod, documented as preliminary in GMP 4, has been removed.
There are a number of compatibility issues between GMP 1 and GMP 2 that of course also apply when porting applications from GMP 1 to GMP 5. Please see the GMP 2 manual for details.