You can use libfb from C, Gambas3 to connect Firebird Database Server
C Library to connect and use Firebird SQL Server Database. This library wraps firebird's C API and provide helpful functions to execute database queries. It includes a Gambas 3 port.
libfb++ is an Open Source project supporting C++ application developers
with technologies for high-performance native Firebird, Interbase database access.
All of the software is distributed under the Lesser Gnu Public License (LGPL),
so you may use it