./configure would be a local script (hence the ./ - which means "in the current directory") - if there is such a file, it's something that would be provided by the software you are trying to build, not by a system package.
In this case, the gambas3-3.0.0.tar.bz2 tarball doesn't contain such a script, but it does contain a configure.ac file. That means that you need to go back one step, and generate the configure script from the configure.ac script using autoconf.
The tarball appears to provide two additional autoconf wrapper scripts called reconf and reconf-all - I don't know which you should use, but I'd guess there's no harm in going the whole hog and running the latter. So the full sequence of commands becomes:
./reconf-all
./configure
make
followed by sudo make install if everything is successful and you want to go ahead and install the software in the default locations (usually /usr/local for this kind of installation).