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.
I still find that even with Midtronics equipment you really need to start with a baseline. Once you have a baseline on a "broken in" bank future tests are far more telling in tracking actual changes to the battery.
Problem with a 20 hour test is that it has to be performed as a 20 hour test and at 77-80F. I built my own tester using a Victron monitor, Omron cube relay and constant current DC load.
*Bring battery to 77F
*Charge battery to full
*Allow battery to rest for 24 hours
*Program & reset Battery monitor
*Apply 20 hour load for battery (100Ah battery = 5A, 125Ah battery = 6.25A)
*Set Victron internal relay to cut test at 10.5V
*Read Ah's removed from battery
Anything less than a true 20 hour capacity test is only an approximation
. I do prefer a Midtronics with "baseline" as it is much easier to track performance but it NEVER lines up with Ah capcity. Problem with any analyzer without a baseline is that many batteries produce better CCA or MCA than what they are rated for when new and broken in.
Testing batteries for capacity when cold is going to be inaccurate as Ah capacity goes down when cold. The 20 hour load will actually be higher to the battery capacity than it would be at 77F. In other words that 5A load at 30F is more like a 10A load at 77F.
Always best to mimic the industry test which is Ah capacity divided by 20 = load, until the bank reaches 10.5V. So (100Ah / 20 = 5A load). It should go 20 hours before hitting 10.5V if the battery is hitting its rated capacity. Counting the Ah's consumed before 10.5V is attained will tell you the Ah capacity as the battery sits currently.
I like my Midtronics & Argus but it is not an accurate representation of Ah capacity..