Which onboard battery charger?

Gel batteries are 'dry' lead acid batteries, so no need to replace the existing charger

Gel batteries are one of the many "flavours" or types of lead-acid batteries.

Different types of lead acid batteries require different charging profiles to get the best life out of them.

You need to find the specs for the charger and compare them with the charging specs for the batteries you have bought.

The charger will undoubtedly work, but it may not be ideal. Too much current or wrong voltage, especially if it is too high, can lead to reduced gel battery life.

If you decide to replace the charger find one that has a gel setting.

Cheers

Gel batteries have specific charger requirements as Frank pointed out.
Some GEL batteries only like maximum charge voltages of around 14.1V and a charger that can handle this such as the Projecta intelli chargers have a gel setting. Some gel batteries can handle 14.4V and the usual AGM seting can be used for these, check your batteries specs. Also worth checking out the float voltage too.

Would also pay to make sure you get a charger of decent charge output..Say 25 or 40 amps for the 2 x 150 amp/hour batteries.Check the old charger and see what its output is. The new chargers have a switch for selecting the type of battery you wish to charge.