According to this report, they have started arresting people suspected to be gays in the Northern part of the country. Here's an article I found on UK Guardian. Read below
Dozens of gay men are reported to have been arrested across northern
Nigeria as police begin to enforce punitive new laws that criminalize
same-sex marriages and membership of gay rights organizations.
Dorothy Aken'Ova, executive director of the country's International Centre for Reproductive Health and Sexual Rights, said that the legislation, hailed the "Jail the Gays" law, had led to mass arrests. Police in Bauchi state, she claimed, had a list of 168 purportedly gay men, of whom 38 had been arrested.