Bower-installer was written as a command-line app and doesn't yet expose an API. You could just use exec if you want to:
var exec = require('child_process').exec;
gulp.task('task', function (cb) {
exec('bower-installer', function (err, stdout, stderr) {
console.log(stdout);
console.log(stderr);
cb(err);
});
})
Or there is a more fancy gulp-exec plugin available. Ideally though, I wouldn't use bower-installer with gulp, but rather use a separate project like gulp-bower-files