How to implement "autoincrement" on Google AppEngine
If you absolutely have to have sequentially increasing numbers with no gaps, you'll need to use a single entity, which you update in a transaction  to 'consume' each new number. You'll be limited, in practice, to about 1-5 numbers generated per second - which sounds like it'll be fine for your requirements.
I have to label something in a "strong monotone increasing" fashion. Be it Invoice Numbers, shipping label numbers or the like.

A number MUST NOT BE used twice
Every number SHOULD BE used when exactly all smaller numbers have been used (no holes).

Fancy way of saying: I need to count 1,2,3,4 ...
The number Space I have available are typically 100.000 numbers and I need perhaps 1000 a day.
I know this is a hard Problem in distributed systems and often we are much better of with GUIDs. But in this case for legal reasons I need "traditional numbering".
Can this be implemented on Google AppEngine (preferably in Python)?