No trouble sharing the gateways.
The only trouble is if the gateway in china is very busy (all the channels are busy most of the time because the carrier is using every available channel).
in that case, you may send a call from USA to china and get a “no circuit avaialable”.
A way around could be leaving one channel out of the group for this type of calls.
Post your e-mail and I will contact you about the details.