None of the global_conf.json files for US 915 containTX LUT entries.
Semtech has not yet merged the TX LUT entries.
See here
Could you create an issue here to make sure we keep track of this: https://github.com/brocaar/lora-gateway-os/ ?
Even better, would you be able to create a pull-request? 
I must confess my ignorance here about what or how to do a pull-request.
I am willing to learn.
There is an open pull-request in Lora-net/packet_forwarder for this here
How would I duplicate that?.