There was a problem where DG4ODBC was included on the install media for some platforms where it is not actually supported and it looks like Solaris x86 is one of those.
There is no official port of DG4ODBC to this platform.
There's an official certification matrix available on OTN:http://www.oracle.com/technetwork/database/gateways/certmatrix-168347.pdf
=> it shows Solaris SPARC as certified platform not x86.
So although it is linked on Solaris x86 this platform it is not supported nor is there a guarantee it will work.
As it says in the earlier updates the gateway products are not available on Solaris x86(64bits) and are only available on Solaris Sparc 64-bit.
As far as I know there are no plans to port to Solaris x86(64bits). The only workround is to install the gateway on a supported platform and access the gateway from the RDBMS on Solaris x86(64bits) using database links.