void take_value (Value v)
{
}

void main ()
{
	take_value("blabla");
}

This generates the C code:

void _vala_main (void) {
	GValue _tmp1_;
	GValue _tmp0_ = {0};
	take_value ((_tmp1_ = (g_value_init (&_tmp0_, G_TYPE_STRING), g_value_set_string (&_tmp0_, "blabla"), _tmp0_), &_tmp1_));
}

_tmp0_ (or _tmp1_) should be freed here (g_value_unset()).