A good place to start would be some simple command line tricks. You won't need to install anything just open up a unix terminal.
To see how many gene features are in your gff:
awk '$3=="gene"' your_file.gff | wc -l
This gives you the total number of gene features. You can play around with this command to view other features like "transcript". Also you can look at
$5 columns which are the start and end of each feature.