WiMax isnt GSM, it runs on a totally different frequency, one area na mataas ang GSM, UMTS/HSPA+ are might have low to non existent covarage on WiMax, same applies with LTE since that runs on a different frequency
though much like GSM its possible that there is high covereage of WiMax and another area low to non-existent
WiMax is a DEAD technology killed by LTE, it was a war like HD-DVD and Bluray had back in the day which bluray won, nanghihinayang na lang ang mga companies na naginvest sa WiMax tech and Deployment sa lahat ng ginastos nila kaya ginagamit pa rin and they still have service, but unfortunately WiMax will no longer evolve or expand coverage to how much it is right now since LTE and LTE-A is the future of wireless Broadband
another reason why WiMax DIED is Mobile devices, most new models of phone and tablets already have LTE built in(like most Qualcomm Powered phones have LTE in the CPU DIE) and no Manufacturer uses WiMax for 4G on the devices they make