GmailDefaultMaker is described as 'Gmail as the default mailto: client'. There are three alternatives to GmailDefaultMaker for Windows. The best alternative is EasyMail for Gmail, which is free. Other great apps like GmailDefaultMaker are TVHGoogleMapi (Free, Open Source) and Affixa (Free Personal).
When you move to gmail or google apps you can no longer use the build in 'send to mail recipient as attachment' menu option of MS Office and other windows applications without installing a local mail client.
Affixa makes your webmail account part of Windows. Affixa makes gathering and preparing the files you want to send easy. Affixa gives you the choice of which web or desktop mail system you use and when.