There is fantastic news about Google Photos. With Event Gallery 3.10 they'll work even after January 2019. The process of getting Google Photos is slightly more complicated, but I made a tutorial video to get you started.
The new API is slower than the old one. It required multiple requests to get all album and image data. You can just get 50 or 100 items per request. An album with 1000 pictures needs 10 requests. This sums up very fast. It is clear that you can't go through that process for every page your server delivers. The trick here is caching and asynchronous loading. Caching happens on different layers. Initially, the database needs to be populated with image data. The response of the request to the Google service if cached for 24h and configurable in the components options. The album selection in the back end will show albums. This data is cached for 5 minutes. If you add a new album, you need to wait at most 5 minutes for your album to appear. The last layer is the information about image URLs. They are valid for up to 60 minutes, but you never know when the clock started ticking. Because of that, the cache time for thumbnail requests is set to 10 minutes. If you want to clear that cache, you can delete the folder com_eventgallery_googlephotos in the Joomla! cache folder or use the cache clearing backend functionality. But be aware that refreshing the whole cache will take some time in which your site responds slowly.
The is no migration path from the old Google Photos implementation to the new one. Both use completely different identifiers which I can't match. To use the new Google Photos implementation, please delete your old Google Photos events/albums and create the new ones with the new folder type.
The Google Photos API has a limit of 10.000 requests per day. You can check your API consumption on https://console.developers.google.com. This page will clearly show how much traffic you use. If you exceed that limitation, there might be a solution. The limit applies to an API project. Event Gallery supports multiple projects to load your images from Google Photos. As a workaround, you can create another API project in the Google Console and add a new Google Photos Account in Event Gallery. You can also try to increase your API limit, but that required validation of your application. Check the acceptable use policy here: Acceptable use policy.
Right now it is not possible to share or sell pictures hosted on Google Photos. The thumbnail links are valid for just 60 minutes. If you need this features, we can talk about the implementation later.