Sometimes we need a spacetime which is not predefined.
>makeg (metric):
Then one can enter the metric 
or the line element
, a non-holonomic basis or a null tetrad.
The use of this facility is easy, one just have to follow the
instructions.
To reuse a metric that you save you use
>grload ( metric, '/dir/metric.mpl' ):where '/dir/metric.mpl' is the file it is saved in.