Grooming, in this context, means "The act of gaining the trust of a minor [or other vulnerable person] with the intention of luring them into an abusive sexual relationship."
Yes, it is a law enforcement issue.
The Prime Minister, as Head of the Executive, is responsible (in discussion with Cabinet) for setting policy on law enforcement. He can direct the NCA (a law enforcement agency) to create a task force to focus on a particular category of crime that is of concern. There are task forces on border security, gun crime, modern slavery and many more. As you say, this should be handled by a law enforcement team. That is exactly what Sunak is proposing. (In fact, such a team already exists so the "I'm creating" is a bit of political hyperbole)
The particular context are crimes committed by groups such as this gang from Bradford. Note that "gang" here is a criminal conspiracy, and not like the "Color gangs" or "MS-13" in the USA. There have been many of these "grooming gangs" particularly in the major cities of the North of England.
Be sure that you understand "grooming gang". It isn't the name of one particular gang, it is a description of various criminal conspiracies in the UK.
Now Sunak talks a lot about "political correctness". This is because many of these gangs, such as the Rotherham gang, were associations of South Asian men. In the Rotherham case, it seemed that police officers had failed to investigate properly, did not recognise evidence of abuse. At least in part this seems to have been a result of the fear of being accused of racism. This historic failure adds to the political nature of the enforcement of these crimes.
For background into the Rotherham gang, and the police investigation, you can read the Jay Report.