Update:  This solution doesn’t really work. Prism doesn’t start up without a network. So if you are offline, you don’t get to see anything. If you are online (connected to a network) though it works. I will update this post if I find a solution.

Here are the instructions to get Gmail Offline working with Prism on a 64 bit installation of Ubuntu 9.04. I am assuming you have already installed the package prism-google-mail. If not, go to Synaptic Package Manager and install it.

1. Download the 64 bit version of Google Gears found here. (There are 3 links; I used the last one : gears-linux-opt-05210.xpi). Download it with another browser like Opera, so that the install dialog does not pop up.

2. Open the downloaded XPI file with the Archive Manager (File roller). (It’s just a matter of right clicking and extract. )

3. Edit the install.rdf fil. Search for code that looks like :

<em:targetApplication>
<Description>
<em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
<em:minVersion>1.5</em:minVersion>
<em:maxVersion>3.0.*</em:maxVersion>
</Description>
</em:targetApplication>

and replace it with this :

<em:targetApplication>
<Description>
<em:id>prism@developer.mozilla.org</em:id>
<em:minVersion>0.1.*</em:minVersion>
<em:maxVersion>10.0.*</em:maxVersion>
</Description>
</em:targetApplication>

4. Save the file. Archive Manager will asking you whether you want to update the archive. Say Yes.

5. Open Prism – Google Mail and on the bottom right corner click the Settings button and navigate to Tools > Addons.

6. Drag and drop the modified XPI file into the Addons window.

7. That’s it.

I have combined instructions from here and here to  create this post.