I concur with Protect and Stoneskin being the best Conjurer abilities to cross-class with Gladiator and Paladin.

Cure can be also useful, especially used with the native GLA ability Convalescence (which boosts the potency of healing spells cast on you), but later on especially it becomes rather impotent for proper healing, so a lot of players don't bother with it (although when soloing it can still be a help). Raise can be useful too in a backup situation, or if you come across someone KO'd and there's no CNJ/WHM/ACN/SMN/SCH around to raise them.

Stoneskin and Protect are pretty much the most useful spells Conjurer gives to Gladiator and Paladin though.