1) It has a very very strong integration with Microsoft Excel and it's the primary runway from where Gemini takes it's flight to the Business Intelligence space. It is also heard to have nice integration with Sharepoint and other report authoring or rendering tools.
2) It's heard to be an in-memory storage form of Analysis services, i.e. it can be said as an another compound of analysis services. In fact data stored in Excel via Gemini add-in for excel is stored in the form of a .AS database, which means that it can be consumed by any applications which can consume data from analysis services. I may not be precise in the above statement, but the summary is data projected by gemini to its host applications can be or rather would be in the form of an analysis services database.
3) Gemini add-in for Excel is one of the fantastic ways for business users to get the most out of Excel, still having the capabilities of the likes of any data warehouse analysis hosted in a dashboard hosting environment like performance point. Here Gemini is the analysis tool, Excel can store the data in place of data warehouse and Excel charts / graphs act as the dashboard or visualization holders. Also Gemini add-in makes use of Silverlight for it's visualizations.
4) This tool as heard and discussed by industry experts, is a Microsoft's competition to tools like QlikView in the self service or I would like to use the word "portable" business intelligence arena for business (non technical) users. The advantage that gemini has, is the immense propagation of Microsoft Excel in the industry with its added popularity among business users, and its tight integration with the same.
5) Below is a link of few videos which provides preview of project Gemini: