Pitch Black Fire Wizard

Gerard obviously knew. He knew all that transcribed the moment he was teleported to the peak of the mountain in Amethyst City and confront Mammon was the Goddess Daikokuten's Will and intentions.

She used the opportunity of being indebted to 'Her' by saving him from being totally influenced by Mammon and becoming a cold-hearted killer like Everett before he was confronted by Kurosawa.

It would be a lie if he said he didn't care. But having been around with the Goddess multiple times, he had a rough idea of 'Her' personality which helped him to understand the underlying meaning behind 'Her' words.

The real reason why the Goddess urges him to face Mammon head-on is the insurance 'She' was betting on. Asmodeus, another Prince of Hell. So even if Mammon tried to kill him aside contrary to 'His' words about preserving his life so Asmodeus can descend once the latter's influence meets the condition for 'Him' to ascend.