CB Gallery capacity:
Can CB Gallery handle huge amount of pictures? We would like to set up in hundreds of Gallery folders over 25 thousands images. And after the community will increase the amount.
Can CB Gallery deal with it?

It absolutely can. CB Gallery has been performance tested with over 1 million gallery entries and albums.

Is there any technical capacity that we might aware of?

Your only issue maybe server storage capacity. CB Gallery currently does not support external storage so you'll need to be sure your server can handle storing that much data.

CB Gallery GoogleDrive links:
It is not supported now. Do you plan in future CB Gallery allows link GoogleDrive links?

We do not currently support external storage. External storage methods will be explored in the, unfortunately distant, future for CB it self. The plan is to implement a generic implementation in CB that every plugin would then also utilize. As for what external storage would be supported we'd likely support Google Drive and Amazon AWS at the least.

Would it be a hard customization for a developer?

That depends on if you mean an actual web developer that knows how to code then no it shouldn't be difficult. There would be some learning curve if they're not experienced with Joomla or CB at all though. All of the storage code and display code would likely need to be changed to handle this.