I'm not aware of a tool that does what you need (but it might exists nonetheless). I guess you need to do some scripting, and essentially I would approach this as intron_length = transcript_length - (sum of exons and UTRs). As you can see the third field contains features such as 'exon', 'gene', 'transcript', and the last field contains information about the 'parent' transcript and gene a feature belongs to.