How do I pick a suitable battery for my sensor?
Two 6V 3.5Ah batteries connected in series is equivalent to a 12V 3.5Ah.
The available capacity of the gel battery and the voltage after discharge, at different discharge rates, would be as shown.

It follows that the load could have been in the region of 2.5A (assuming the batteries carried 100% charge to start with). This may be confirmed by measuring the sensor current using a 12V DC power supply or using the same batteries after a recharge.
Were that to be confirmed, a 12V 18Ah gel battery would be just right for this application.
In my opinion, a 12V DC power supply would be the way to go.