1) dbaccess cannot use ODBC, so you can't connect to any non informix database with it.
2) Not sure there is a Mysql ready edition for Enterprise gateway
3) depending on what you have to do on the application side:
a) if no human interface is involved, perl with the packages DBI packages for Informix and MySql is probably the best solution
b) if a human interface is involved, use Querix Lycia, which compiles x4GL language, but runs on informix, oracle, sqlserver, mysql and postgresql. I think genero does this too, but it is more expensive.