Update: It appears as though T-Mobile is now pushing the update over-the-air (OTA) as well. You can check manually by heading to Settings, then About phone and check for update.
If you're rocking the T-Mobile variant of the LG G4, model H811, you'll want to check the LG Bridge PC software for the Marshmallow update. While not available as an over-the-air download just yet, there are a number of reports of users being able to download the software from their computer. Once you plug your phone in and check for an update, you should be prompted to download the Marshmallow update.
T-Mobile has not yet updated its support page with information about the update, and at this time it is not known when we may see the update push out as an OTA download. If you want to get Marshmallow on your phone sooner than later, be sure to check the LG Bridge PC software for the update.