In general: You need to trust the machine on which you are decrypting the files. This trust is usually only warranted if you have full control over this machine which also means that you can install software on it. So if you are really concerned about the contents of your files then use an offline encryption/decryption tool on a trusted machine and upload the encrypted data to Google Drive or any other cloud file sharing service.
Then how do you access encrypted data on the library computer? Well, you don't. That machine might have malware on it that sends copies of your data and/or your key to somebody who is not supposed to have them. Once this happens your entire encryption is useless.
This also explains why there are no "online solutions to encrypting Google Drive". I'm not really sure what you mean by "online" here but I am assuming you mean "transparent" i.e. the data is encrypted but you don't really have to care about how it is done and don't need to worry about installing software. Here's why this is so hard to do:
Encryption of data always requires two main ingredients:
- The algorithm used to encrypt the data and
- the key used to encrypt the data.
The key must never be known by an untrustworthy entity. But you need to supply the key to the algorithm and an untrustworthy algorithm might just send the key to an untrustworthy entity. So you want the algorithm to be trustworthy. This, however, means that you should not use an algorithm provided by an untrustworthy source.
Applying this to your Google Drive situation
Since you want to store data so that Google can't read it we are going to define Google as untrustworthy.
Transparent encryption without additional software on the client side would have to be done either on the Google servers or in your browser.
- When it is done on the Google servers it is obvious that you need to supply Google with your encryption key so they can decrypt your data on their servers. So now they have your key which means they can decrypt which means you might as well have stored your data in plain text all along.
- The other option would be decryption in your browser. This, however, would have to be implemented by Google so they can serve it to you on their page. Again, you would have to hand the key to an algorithm that is from Google and that algorithm might send the key back to Google.
A trusted third party
After all that talk about how it is impossible to do transparent Google Drive encryption here comes a way how it might be possible. However, this requires that you trust some other party to see your sensitive data.
Google Drive has an API that allows viewing and uploading of files. In theory it would be possible to build a website that accesses your Google Drive using that API and encrypts and decrypts data before it is written or read. This means that Google would never see your plain data and you would still be able to access it without installing additional software.
However, now you have a different website owner who provides the decryption algorithm and might therefore have your key. And you have also handed him your Google Drive login so now he is the one who will be able to download and decrypt everything in your Google Drive.
The sad conclusion
If you really want to make sure that nobody sees unencrypted data, then you can't show your unencrypted data to anyone. This, however, means that you need to do the encryption on a trusted machine and that usually mandates that you install additional software. Also you can't decrypt data on untrustworthy machines such as library computers.