That’s correct; if you sync a database that contains indexed folders, the folder contents will be available in DTTG. I think a lot of DEVONthink users use this feature. For example, some of us use Zotero to manage academic articles, and then index the Zotero storage folder in DEVONthink to make the PDFs searchable and annotatable in DEVONthink. Thanks to DEVONthink’s sync facilities, the articles are also available in DTTG, and one can use the excellent PDF annotation features to read and mark up the articles on, say, an iPad.
At this point, it’s worth bringing up a difference in behavior here between DEVONthink on the Mac and DTTG on iOS. You will run into this sooner or later, and it’s worth keeping this in mind because it affects how you work with things across multiple devices.
In DTTG on iOS, the indexed folder contents are available even though the external indexed folder itself is not on your iOS device. By contrast, if you have 2 or more Macs running DT, the indexed folder contents are not available unless the external indexed folder is present on all the Macs where you want to access it. DT uploads the full indexed folder contents to the sync store, but DT on the Mac currently doesn’t offer a feature to download index folder contents from the sync store, even though (paradoxically) the contents are obviously there. (I know DEVONtechnologies are considering the addition of the feature, but AFAIK it’s not available yet.)
Typically, the problem you run into is that you have a database with an indexed folder on your “main” Mac, say a desktop iMac where you do most of your work, and then you decide you want to have the same database on your laptop. So you set up DT and sync your databases there, but (accidentally or by choice) you don’t have the source folder on the laptop too. Then you discover that you can’t access the indexed folder contents on the laptop.
The solution is to make sure to have the indexed folder on all Macs where you want to access it.