GST will be charged value of supply. Suppose you have given advertisement worth 50,000 to a news agency and you charged commission of Rs. 5,000 then GST will be charged on 5,000 only.
But if you have received money from customer and then paid to news paper, in that case GST will be charged on full amount by you to customer and newspaper will charge to you.
If advertiser directly pay to news paper and you work as only a commission agent and rise bill for only commission amount then you need to charge GST on commission amount only.