Question: I've recently noticed that most of my activity with Google and its services goes through a local Google Global Cache server set up by our ISP. Since I don't fully trust the ISP, this has become a big concern for me.

Is it possible for the provider to read the encrypted requests that pass through GGC, thus getting access to my mail, documents, search queries and perhaps even password?

Answer: 
As described [here](https://peering.google.com/about/getting_ggc.html):

> 
> Once registered and qualified by Google, we will send you a simple agreement for joining the GGC program. After you have electronically signed this agreement, Google will *ship you servers that you install in your facility* and attach to your network. Google will work with you to configure the servers and bring them into service.
> 
> 
> 

Emphasis is mine. What this means is that your ISP can *potentially* inspect all the Google queries that the GGC is supposed to cache -- but this entails "opening" some tangible servers shipped by Google themselves. I suppose this would be a flagrant breach of the contractual agreement between the ISP and Google, so chances are that your ISP won't do that. Unless Google agrees, of course. Even if your local Law enforcement agencies insist on eavesdropping on you, it is probable that Google will be involved.
Your search queries are definitely very easily accessed as you can see by the [recent DNS interception in Turkey](http://googleonlinesecurity.blogspot.com/2014/03/googles-public-dns-intercepted-in-turkey.html).

How much information they are able to get depends on how extensive the interception is. Your search queries are the easiest for them to see. Use [DuckDuckGo](https://duckduckgo.com/) while signed out of Google in an incognito browser for more secure search queries.

As far as other requests, if "they" really want to see it, I'm sure "they" could get to it other ways, not only through your ISP...