Crossing ATZ above 2000ft - need to call in?

[Conventional Gear]

You do bang on about it, if you are in the transponder mandatory zone all you need is DME tuned into 110.5

If it reads more than 8.0 (8.5 if you worry about slant accuracy etc) you are outside the CTR and no worries.

I like GPS but have navigated this 'Gap' many times without needing GPS