TSS of a gene vs TSS of a transcript
There is no TSS of a gene. Geneic region will be a union of all the transcripts arising from that locus (including introns). 

Sometimes two variants of a gene can have different TSS. You can define a 5' boundary of the geneic region which would be the most 5' TSS amongst all variants. You can also give scores to the TSS based on the expression of the different transcripts.

Nevertheless, I am not sure why you want this.