Re: 13 plus volts on rested battery?
As noted above and elsewhere, the only reliable tests of remaining capacity are either a 20-hour test or the use of a modern (and expensive) tester like the Midtronics series.
Gels are tricky. I've got two golf-cart size gels in my basement shop right now which are pushing 15 years old, the first 10 in use on a sailboat. Been using them for testing and for utility the past five years. One of these still measures more than 90% of original capacity; the other measures a bit lower. I use a Midtronics MDX-650....much easier than the 20-hour tests, but I do those, too.