Gel cells require reduced Charging Voltages, but AGM does not need a different profile than "regular" flooded cell batteries.
The two questions which might justify a new, MPPT-type charge controller are:
#1 (Most important): Are you failing to charge up the batteries to 85-90% maximum SOC with the Solar setup you now have?
If you can fully charge the batteries now, then buying more panels (or a "better" controller) is money wasted on extra "charging power" which you will never need to use.
#2 (Second most important): Do you anticipate a need for more than 250 Watts of "maximum panel power", because you will be camping in non-optimal conditions, or because you consume a lot of power during the daytime?
MPPT controllers improve the efficiency of all the panels by about 20-25%. But they typically cost at least as much as adding another panel. If you have (or will have) less than 200W (nominal "max") on the roof, then your money is almost certainly better spent by simply adding another panel. When you cross over from 200W "max" to 300W "max", then the additional power provided by MPPT could be cost effective.
But when you consider a "need" to upgrade above 300W nominal "max" of all your panels together, than upgrading to MPPT will buy more "power per dollar-spent" than you get by adding yet another panel into a PWM controller.
For example: A PWM controller, charging batteries at 14.4V, will connect and disconnect the Panels very rapidly to make the
average Voltage from the Panels. But a typical "100W Panel" will try to deliver those 100W as 17.9 Volts * 5.59 Amps (BTW, that is a genuine and typical pair of numbers.) A PWM controller will refuse to take in power which the panels could have provided at "excessive", accepting only the operating current (5.59A) at the controller-limited "Average Voltage Desired" (14.4V, or perhaps as little as 14.2V. We'll use 14.4V, that's the best case for PWM.) 14.4V * 5.59 = 80W, the other 20W are never delivered to the battery. A good MPPT controller would convert that 20W into more current at the lower Voltage, minus about 3% efficiency loss, yielding about 97W.
With just one panel, those 17W are insignificant (in comparison to the price of an MPPT controller). But with 4 panels, the PWM-wasted power is approaching 80W. That's nearly an entire panel of unused energy. In different operating modes, the charging Voltage might be only 14.2V (with max current allowed at that Voltage). In that mode, PWM loses another 11W on each panel, and MPPT has delivered about 120W more energy into the batteries (and appliances) than the PWM could do -that's more than an entire panel, but only when the configuration is large.
Unless you will need to upgrade the "80W" panel into more than "200W" on the roof, stay with PWM.