GAAP — or Generally Accepted Accounting Principals — are a set of standards that govern corporate accounting.
Managerial accounting does not have to adhere to GAAP so long as the ad-hoc reports are for internal use only, and not official. However, all financial statements like the Profit & Loss, Balance Sheet, etc must follow GAAP.
This means managerial accounting reports can be used within a company to inform decisions and strategies, but they cannot be submitted as official government documents.