I'm sorry for not specifying but GVMax Pro is the version that doesn't require giving out your credentials -- unlike GVMax Lite. Pro is a one time $25 fee. And as far as the open source goes, he just sent this email which explains new features not yet advertised on the site:

<email>A new version of GVMax (4-0-0) has been released to existing customers (not yet publicly released).

This is a major upgrade that contains many new features which I hope you will like.
Here are some of the major new features:

- Voicemail notifications
You can now get notifications of new voicemails in much the same way you get SMS notifications.
These notifications show the Google transcribed text for the voicemail, as well as providing a
link to a recording of the voicemail.

- Make GoogleVoice calls from within your IM program
In a chat window simply type 'ggc' to initiate a GoogleVoice call to the GVMax contact you are chatting with.
Alternatively type 'ggc {number}' to call the given number via GoogleVoice

- Bulk SMS sending
You can now send an SMS to a group of people.
The members of the group are derived from your GoogleContacts groups.
You can do this from within the IM program and/or via email

- Better error handling and logging

- OpenSource
As of this version GVMax is open source. Hopefully this will get more people involved in adding new features etc.

- Check out the User Manual (a link in the application) for full details of all features.

If you were already using version 3-0-0 then your application should have been upgraded automatically. For those of
you who are still on 2-1-0 I will be contacting you directly with instructions on how to upgrade.

Last week Google released a new version of their SDK and that has introduced some instability to their platform for
applications running on older versions. This meant I have had to bring forward the release of 4-0-0 and
unfortunately have not quite finished polishing it (expect another minor release in a week or so).

The one thing I have not had time to do so far and which might be of interest is how to setup your application
to receive voicemail notifications. Here is what you need to do

In the same way that you created an email filter on your GMail account to forward SMS to your application so will
you need to do for voicemail.

1- Login to GMail
2- Click on 'Settings' (top right of screen)
3- Click on 'Filters' tab
4- Click on 'Create a new filter'
5- In the 'From' box type: voice-noreply@google.com
6- Press the 'Next Step' button
7- Select the 'Forward it to' checkbox and type: vm@{appname}.appspotmail.com (replacing {appname} with your application name).
8- If you don't want a copy of the original email from Google then select the 'Delete It' checkbox
9- Press the 'Create Filter' button

I am hoping to create a screencast detailing this soon, in the mean time feel free to contact me if
you run into problems with this.</email>