Hi @ialberquilla,
If I understand correctly, you want users to sign meta-transactions (signing will be done by your service as you are controlling the keys) that are then relayed with your service paying the transaction fees.
You should be able to use the GSN in this scenario.
All development on the GSN has been moved to the OpenGSN organization.
I recommend asking this question on the OpenGSN Telegram: https://t.me/joinchat/F_BETUjG0Crb2s6mFx1LWA