Coding style:

​GattApplication::GattApplication(QObject *parent)
    : ObjectManager(parent)
    , d(new GattApplicationPrivate())
{
}

const

GattService *service

Should this be made private (preferrably in GattApplicationPrivate)?

Shouldn't the caller be made responsible for choosing object path?
If we force a path then we should use "our" namespace - /org/kde/bluez-qt/.

const QByteArray &value)

Any reason to use uint8?

No additional spaces please.

newline

GattCharacteristic *m_gattCharacteristic

const auto children =  application->children();
for (const auto &child : children()

Also you can use findChildren<GattService*> as above.

This should have d-ptr as it's exported class.

Probably would be better to add setters for uuid/primary (and note that it can only be set during creation), as if we need in future more properties we will need to add new constructors.
Or make it virtual?

Same as above, add setters?

Will this be used for more classes? Right now only GattApplication inherits it.

Maybe it would be best to add constructor that uses default object path. Also please add better documentation as to what object path prefix is.

explicit GattApplication(QObject *parent = nullptr);

Here I meant adding new constructor, because as you did it now, if you want to set parent from constructor you would need to copy the default value for the objectPathPrefix.

GattApplication(QObject *parent = nullptr);
GattApplication(const QString &objectPathPrefix, QObject *parent = nullptr);

I don't really like this class at all. This is implementation detail, and is of no use for the users of the library, so it shouldn't be exported. On top of it, there is actually no code, it is just interface class.

Another issue here, although minor, is that the bluezqt_dbustypes.h is not installed, so you can't use it. Yes, you could install it, but we don't need it.

As I can see, the only reason for this class is so you can pass it to ObjectManagerAdaptor, but there are other ways to achieve the same thing without adding ObjectManager and deriving from it in GattApplication. If we really need it in generic way, we can do it later.

Something like this would work:

ObjectManagerAdaptor(QObject *parent)
{
    GattApplication *app = qobject_cast<GattApplication*>(parent);
}

DBusManagerStruct GetManagedObjects()
{
    return app->d->getManagedObjects();
}

Or for now just make it work only with GattApplication, as this is the only one we have now.

Yes, it will not work in the autotest, since you won't have the hook anymore, but you can just remove that test.