As mentioned, you'll need to get Conjurer up to at least level 15 to unlock Paladin, but in addition, you should also unlock Marauder and level it up a little for some useful cross-class skills. Within just the first eight levels of MRD, you'll gain access to four cross-class-able skills useful to your Gladiator/Paladin. Then there's another once you get MRD up to 26, but particularly those first four are nice low-hanging fruit so to speak, as it doesn't take long to get a new class up to level 8.