Hi, your question is great. No clear cut answer. I haven't dealt with on-call GI nursing for 7 years. So my knowledge may be out of date. And fortunately I was only peripherally involved in a bad GI bleeder on off hours. It was a real SNAFU, but the patient survived.
Of course few nurses (or at least not me) can feel competent to deal with an after hour emergency (like a GI bleeder, Sengstaken Blakemore tube, etc.) if you are only on call once every few months!
Ask for (demand) an in-service on GI bleeders or other typical on-call emergency situations. Ask to shadow a nurse when there is an emergency case before you are expected to take call. Have a mock model of the Sengstaken Blakemore set up, make it as realistic as you can. You can "play" with it during normal hours.
I don't know if you have the option to refuse call. I am not encouraging you to do that, but I know it can be scary. My nursing philosophy has been grit you teeth, be scared to death, and dive in.