IMHO
I would say it depends on how 'old' they are.
Some of the charge systems are almost designed to ruin the batteries in 3 years.
I would ask to check the charge system with a meter. If the batteries in it now are over 13.7, the old ones may be toast. I don't think they will hold up.
Ask why they were replaced and how long they were in service. There is a date code on them somewhere, finding and decoding it can be hard. There may be a service sticker with date in the unit.
I would pass if the charger had them over 13.7, or if they are over 4 years old.
Gels need a lower charge V than many controllers have. Check if your solar controls are compatable. Some have switches or jumpers to adjust for the battery type.
What are they going to do with them anyway? You may be the only guy who has any interest. Not a big (or small) market for those things most places. They will probably end up hidden in a dumpster if you don't take them. Sad but true.
G-