It has to be Havoc if your aiming GS at semi-pro developers for 2 main reasons:

- It's a good, well known engine, also gives you extra clout with publishers; it really is a well known brand.
- Its that well known for a good reason, its a very good physics engine, it has wide support accross a large number of 3rd party tools available out there.

Of course it comes with the major downside that it can be expensive and that cost would likely be born out to the end user, so most likely this is not a viable option.

Therefore I would cast my vote to Newton simply because it has been a part of GS for a long time, all be it, as a 3rd person tool/plugin. So if any engine has earned its inclusion with GS it's that one.

Other than that, the floor is very much open

Just my random £0.02 for the day