Using gff format in IGV.
1. Do not omit the first line, especially for GFF 3.
As IGV supports both of GFF ver 2 and GFF ver 3, the file needs to be distinguished.
Do not miss the first line
2. Make sure tab is used to separate fields.
Even for the first line, there must be a tab between gff-version and 3.
Here are the specifications.
GFF version 2
GFF version 3