Do girls parents have to give dowry( it's a custom practised in india, Girls parents present boys with money property gold etc at the time of marriage)

Because of this many poor parents are not able to get their daughters married,because they can't afford it?

It's hard to answer a question about very old customs that are still around, despite the modern world. Of course 'have to' would have to be defined... if a boy wants to marry a girl for love, and her parents can't pay a dowry, and his parents don't approve, then he and she have to decide what to do on their own.
Or are you asking about the law (in India).. I think it's against the law to demand a dowry... although I do believe that sometimes the man's parents have been known to demand it at the wedding, using shame to see what they can get.
Plenty of poor girls marry in India, don't they - if the boy is equally poor, his family can't make many demands or he will never find a wife.