You can't use the reflexive triggered ability to try to attach an Equipment to a creature if that Equipment can't legally be attached to that creature.

If you put an Equipment with job select onto the battlefield with Gilgamesh's ability, the job select ability and the reflexive triggered ability will both trigger, and you'll choose the order in which those abilities go on the stack. For example, if you put the reflexive triggered ability on the top of the stack with the job select ability below it, the reflexive triggered ability will resolve first and allow you to attach the Equipment to a Samurai you control. After that, regardless of whether or not you chose to attach the Equipment to a Samurai you control, the job select ability will create a Hero token and attach the Equipment to the Hero token.