GDPR should help but if the apps were hosted on a developer account registered to a business entity, the majority of data will relate to the business rather than the individual, in which case GDPR may not throw up a huge amount. I don't believe the automated decision making parts of GDPR apply to businesses either.

In the UK at least, if it wasn't a registered business (limited company) then there is no legal distinction between you and the trading name and hence all the rights under GDPR would be available to you.