BTW, decided on the FT-897d (100w). I would be using the rig for receiving on the average 1-2 hours per day, little to no TX.
First thing is to differentiate between GEL and AGM. AGM is what you want, as gel-cell's are commonly mistaken for them. Popular in the 70's, but not today, and especially not for solar since they need to be slow-charged on purpose so as not to develop voids in the electrolyte...
AGM is the modern replacement. Unless you totally abuse them, they are safe indoors since they don't vent hydrogen. Again, under normal charging conditions.
Let's say the 897 draws 1A on rx. You want two hours a day. That's 2ah daily. Easy.
1) You don't want to take a battery past 50% DOD or Depth Of Discharge especially if you are using it daily. So... that means you require at least a 4ah AGM. Let's make that 5ah since that is easier to find, like a Powersonic, UPG, B&B etc.
But what is 50% DOD for a small agm - how do you measure it? After 4 hours of no-charge and no-load, measure the battery terminals. Don't let it go below 12.2 volts. Under your 897's small load, I'd say don't let it drop below 12.0 volts as shown in the 897's voltage display. Note that the 897 voltage display may differ from a quality voltmeter, but unless you are using something like a Fluke 87V or better, the 897's voltmeter will be in the ballpark.
2) Small agm's like this can only handle 0.25 to 0.3C max charge current. That means the largest current you can hit it with is 1.5A. (0.3 * 5ah)
3) For solar, a small 20 to 30 watt panel will suffice, with charge controller. A "nominal 12v solar panel" usually has an ocv of anywhere from 17-21 volts or so. Let's say 18v. 20w / 18v = 1.1A. That would be fine. A slightly larger 30 watt panel would put out 1.6A. Just barely exceeding a 5ah max 0.3C charge current battery limitation. (C = the ah rating of the battery)
4) A suitable charge controller would be something like a small Morningstar Sunsaver SS-6-12V pwm. It has two choices for charge voltage, "sealed" and "flooded". Even though AGM's are sealed batteries, you want to select the "flooded" option, as this is the correct VOLTAGE for agm's. What Morningstar calls "sealed" is actually intended for GEL's, which need a lower voltage, and is not a chemistry we want for solar. Essentially, go by the proper voltage - in this case "flooded" is about 14.4v charge. Small agm's like 14.4, and some even 14.7 or so. Basically what I'm saying is to purposely remove the jumper so that you are feeding it the "flooded" voltage for your small agm to let it charge properly. The Morningstar will eventually drop to 13.8v float when the battery is full.
5) Make sure you have at least 2 hours of "solar insolation" per day to get the battery charged up properly from your use the day before. Visible daylight is not the same as solar insolation. Basically, we are talking about the typical hours between 10am and 2pm for the biggest benefit. Early morning and late afternoon hours have weak solar output typically. There are charts for this based on your location, but 2 hours should be doable nearly everywhere.
Place the battery and charge controller close to the solar panel! If you run 100 feet of cabling back to your battery from the panel, you'll suffer huge voltage drops and the battery won't charge properly - unless you run something like #00 wire, which would be ridiculous for your basic rx-only application.
Unless you are transmitting with heavy power draw, a small agm battery and small panel would work out fine - just do the math in the first few steps according to the battery size you need. Keep a small agm-specific charger on hand like a Battery-Minder or Battery Tender that has temperature compensation built in as you don't want to let a battery sit around half-charged for weeks on end. AGM's have very little self-discharge, so if you don't use it often, just charge it fully and then top it off every 1 or 2 months.
If you are transmitting, then you should consider the bigger dual-purpose AGM's like an Optima yellow or blue top, but for now, have fun with a small 5-7ah agm battery and 20-30 watt panel and a morningstar sunsaver controller.