If you configure the outputs.conf to use the GTM as the server, this should accomplish what you want.
All all the indexers in the same datacenter? My sense from your setup is that you have a pair of indexers in two datacenters, in which case you will want the forwarders to only forward to the appropriate LTM.
I had tried configuring the gtm in outputs.conf and forwarder started sending data to one of the indexers.
And then I brought down that very specific indexer to check if forwarder will start sending data to one of other available indexer.
It did not. splunkd.log was telling cannot connect to the indexer (which i brought down)