If your first flight were AMERICAN AIRLINES, then you are allowed 2 checked bags. Since yours is US Airways, that may not be the case. Gulf air has complicated policies for Codeshare flights that are not American Airlines.

Its best to call Gulf Air to see if their codeshare alliance includes US Airways or not.

Here is link to the baggage allowance policies for Gulf Air.
http://www.gulfair.com/English/info/Baggage/Pages/Baggage_Allowances.aspx

The sections of interest to you are following (Note American Airlines gets treated differently than the rest).....

TRAVELLING WITH OTHER AIRLINES ON YOUR GULF AIR TICKET

Gulf Air Codeshare flights operated by American Airlines to and from the USA or Canada, have a special piece concept applicable

Gulf Air tickets issued for travel on other Airlines to/from the USA and Canada