While it was updated to support voice and images in fall 2023, ChatGPT-4, like its name suggests, started out based around its central text input, while Gemini and its app have been designed as a multimodal LLM from the get-go.
This explains why ChatGPT's initial training cost might have been lower.
On the other hand, Gemini's general focus on app delivery - for example, prompting users to snap pictures with their smartphones, pick out features in them and have them analyzed - could have warranted a higher cost.