It is haram to develop an app for gambling. Gambling is prohibited in Islam, so developing an application that would facilitate it is also not allowed.
In Quran, Allah (SWT) mentions gambling as one of the things that He has made haram:
“…O you who believe, intoxicants, and gambling, sacrificing for idols, and divination are an abomination of Satan’s handiwork. Avoid it so that you may be successful.” (Quran:5:90)
Even if you don't promote or advertise gambling, you're still doing something haram, which is creating the platform for gambling.
However, if you don't do it someone else will, but the money you earn from it will not be halal. Hence, it's better to find an alternate way of earning money. Being an app developer is a noble profession, so it's better to develop apps that will benefit society.