I'll take a shot at this one. First...if they are indeed gels and not AGM's...they are the MOST tolerant of complete discharge of any battery
type (flooded/AGM/Gel) and since this was a single
event...the harm done may be minimal. On the downside...most yards will simply apply standard battery
chargers of the automotive type to recover batteries. I am hoping they simply plugged in your boat and let your properly configured 3 stage charger
deliver the right charge to your gels.
Your yard should have a dummy load battery tester that can stress your fully charged bank to see what kind of shape they are in. The will need a LARGE amp load model since your bank is large...say around 200 amps.
Like this one which can be had at grainger.com
Absent this type of load testing it is very difficlult to tell the actual damage done to your batteries. Fully charging
them and then leaving them OVERNIGHT with no load
on them will tell a bit...if the voltmeter reads less than 12.6V the next day...they have been damaged. If you run a big house load like a windlass
until the V gets down around 11.8...and the batteries don't bounce back in short order to close to the start voltage, they are damaged. I'm sure you have lost
some capacity over 5 years use but perhaps there is still more life in them. Good luck!