I have irritated white marginal gingiva lingually and erythmatous bulbous gingiva labially after using bleaching gel on semi-custom silicone bite tray?

Stop: Stop using trays/gel. Return to your Dentist for reevaluation. You are having an adverse reaction to either the gel or the trays themselves.

Chemical Burns ?: Bleaching gel can burn your soft tissues if excess material exudes out from under the trays. Custom fit trays are always better.

What to do: See your dentist for an evaluation as to cause and solution. In the meantime stop what you are doing. I'll assume your own dentist did not make the tray and supply the bleaching gel and if that's the case that is why we do not recommend non-professionally supervised tooth whitening products and procedures.