GFF files are definitely closed intervals. The default value of isRightOpen should be FALSE, not TRUE.
Here's one reference: https://www.broadinstitute.org/igv/GFF
> Start and end positions are identified using a one-based index. The end position is included. For example, setting start-end to 1-2 describes two bases, the first and second in the sequence.
I found this previous post on a similar topic: gff files: how to tell if right-open interval convention used?
Thanks,
Shaun
Sounds like this discussion has already happened. You could use rtracklayer::import() instead, it does the right thing.
Thanks, Nico!