starting in Gib
 

I have tried looking for answers, it might have been posted before, but if I arrive on the single BA flight into gib, can I make it back on the same plane, do in and out bound passengers mix? as they do at ARN.

I believe that GB Airways which operated the LHR-GIB flight for BA, has been sold to easyJet so there is no oneworld airline operating there.

Partially correct. GB Airways which formerly operated the flights was purchased by easyjet and, as of March 29, 2008, is no longer a BA franchisee. But BA started flying there on March 30, 2008 and so oneworld flights continue.

Iberia also started MAD-GIB and fly it twice a week.

I stand corrected. Thanks for the info. Since GIB is no longer a "cheap" place to start RTWs, I lost interest in it. It's an unbelievably boring destination.

gib is cheap, £1000 cheaper than the Uk for BOWE4. But its a real pain if you have to stay a night if you cant get the same plane you arrived on back to the Uk

Consider JNB
 

Compare GIB with JNB:

GIB = £3,100 for DONE3
JNB = £2,576 for DONE4
JNB = £3,352 for AONE4

Use a BA one-way award LHR-JNB and then travel First Class going east for only £252 more; if necessary, throw away the final segment (LHR-JNB) of the RTW.