HughesNet sales calls are generally available for review for around three or four months. If the call is available for review and it's determined that appropriate expectations were not set, you may very well have recourse regarding the Early Termination Fee when cancelling. Sales call reviews can take up to a week to complete.
Concerning the gift card, did you go to the HughesNet Rebate Center at the time and fill out the appropriate rebate form for the gift card?
Though it sounds like you'll be cancelling, if you'd like to troubleshoot what it is that's using your data so quickly you can start a new topic in tech support and ask for help with this. There are many things that can use data, including apps/programs on devices that may be running and using the data without you being aware of it.
The reps are on M-F from approximately 9AM to 6PM EST. They usually reply within a working day, so hopefully you'll receive a reply tomorrow.
Thank you for posting and welcome to the community. I am sorry to hear this. I will investigate the sales call and let you know of my findings via private message. Thank you for your patience in advance.