Does the Order of Gems in Your Gemfile Make a Difference

What is the difference between ~ and = when specifying rubygem in Gemfile?

That's a pessimistic version constraint. RubyGems will increment the last digit in the version provided and use that until it reaches a maximum version. So ~>0.8.5 is semantically equivalent to:

gem "cucumber", ">=0.8.5", "<0.9.0"

The easy way to think about it is that you're okay with the last digit incrementing to some arbitrary value, but the ones preceding it in the string cannot be greater than what you provided. Thus for ~>0.8.5, any value is acceptable for the third digit (the 5) provided that it is greater than or equal to 5, but the leading 0.8 must be "0.8".

You might do this, for example, if you think that the 0.9 version is going to implement some breaking changes, but you know the entire 0.8.x release series is just bugfixes.

However, simply using ">=0.8.5" would indicate that any version later than (or equal to) 0.8.5 is acceptable. There is no upper bound.

Different versions of the same gem in Gemfile - possible?

No, It is disallowed in bundler

Make bundler use different gems for different platforms

This is a known issue in Bundler. The workarounds are either:

Generate a Gemfile.lock on a system similar enough to your production environment that you get results that match your production platform. Effectively, that means you can only generate the Gemfile.lock file on Windows if your production system is Windows.
Don't commit a Gemfile.lock file at all, and determine dependencies on the production machine at deploy time (bundle install without --deploy). While not recommended in general, this is an often-used workaround until the bug is fixed. For example, this is the recommended solution offered by Heroku.
Switch to JRuby, which would have the same platform string across both Windows and Linux (java). I don't recommend this seriously, but I believe it would fix the problem.
Fix the problem in the Bundler source code, i.e., help the Bundler team fix the bug. :)

Difference between bundle and gem install?

I'd say: by default on your local machine, no particular difference but...

The purpose of bundle install is to setup everything for the application containing the Gemfile. You can even pass arguments to make needed gems installed in whatever folder you want.

This way in production, you have clearly separated apps with their own gems.

On the other side, gem install gmaps4rails (easy advertisement) gets the gem installed for your whole environment.

Can you have multiple versions of a gem in a Gemfile?

You can set an intervall of allowed gems

gem 'rack', '<1.3.3', '>1.2.4'

It will load the most actual one inside the selected intervall.

But I don't think you can require different gem versions.
If a gem would be loaded in different versions, each class and module must get it own namespace to avoid to overwrite the methods of the gem.

When do bundler update the gem incase of gems pointed to git repo?

You do not need to change the version inside the gem every time you make a change to it. When using git gems, Gemfile.lock locks to a commit hash rather than the version number. You don't need to specify a version at all.

When you run bundle update mygem and mygem is a git gem, it will update the locked commit hash to the latest available on the branch you have specified (or on master if you have not specified a branch).

What is the difference between bundle and bundle update?

In a nutshell: bundle install handles changes to the Gemfile and
bundle update upgrades gems that are already managed by Bundler.

http://viget.com/extend/bundler-best-practices

Needless to say that bundle and bundle install are the same command, install is the default option for bundle.