We are still experiencing that limitation.

The workflow works great from SFDC to Gainsight; multiple external attendees on an activity in SFDC will sync with multiple attendees activity in Gainsight.

The other way around, the Gainsight activity will only have the first external attendee, and the other attendees would show in the note field once synced in SFDC.

Not sure if it works the same for events as tasks but I have done it at the task level yes. 

You need to load to the Task Relation object, mapping the following; 

SFDC Activity ID → Task ID
SFDC Contact ID → Relation ID
False → Is What < this means you’re linking a contact ID as opposed to an account >

There is also an Event Relation object so perhaps the same would work loading to that?

Thank you, @HollySimmons 

Are you configuring the sync with the rule engine or from the activity sync configuration under the timeline admin settings?

@romain with rules engine. 

I’m actually creating the task with a rule too, but that’s because our GS → SF sync is set to create events but we have some timeline types we instead want to be tasks, and these are the ones we want to link to multiple contacts.

We turn those timeline types off in the sync and create with a rule first; 

Then there’s a second rule that runs after, so it can fetch the ID of the task that’s been created (by merging based on subject and activity date), and then adds the attendee’s to it;
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I fetch the existing Task Relation records to make sure it only creates if the record doesn’t already exist, otherwise it throws an error. 

These rules run multiple times a day each, I just space them by 10 minutes to give the first one time to finish before the second starts. 

If you can use the GS → SF sync to create the event/task first then you would only need the second rule though.