The most common use of gff3 files is to encode features pertaining to a genome annotation, such as gene, three_prime_UTR, mRNA, exon, and so on. A gff3 can encode additional features, such as "chromosome", but it is uncommon. So, to get the length of all chromosomes, you would need to select chromosome on the third column, and the end position - the start position of a chromosome will always be 1. Something like this should work:

awk '{ if($3 == "gene") print $1"\t"$5 }' file.gff3