1994 GMC Jimmy slips in first gear/also car starts up but l

Transmission started slipping in first, once it shifts to 2 trans works find, doesw this mean it need a rebuilt are is there something else I can do to it that I don, t kown about, also car starts up fine but lopes alittle while in park or in gearit drives fine and there is no gas smell while driving and it turns off just fine, the check engine ligth came on when the car first would pop when I try to start it then it would run rougth, i, ve since change the gas filtre and that seems to have hsougth that prolem the ligth still stays on and ive unhook the battery+ but the ligth still is on, so what do you think the problem is?