Outlook uses categories where PI uses tags. So in gSyncit:
1. Open Settings > Pocket Informant Sync > Calendar Sync
2. Select your calendar (I use only one, you may have more and then to repeat these steps) and click on Edit
3. Click on the Categories tab
4. Select the radio button at the bottom that says "Automatically sync Outlook event categories with Pocket Informant event tags".
That sets it up, and should work perfectly with events you add from now on, but you may need to restart the syncing process to get this to work on events you have already synced.
Our advice is to create a new event and sync it to check that it is working for new events before resetting the sync to start again :)