This is fixed for v19.07 due in July.
Meanwhile, to fix your current project, remove all GIFs from the Playlist. Add your GIF by double-clicking it in Recent or using File > Open… At this point you may be prompted to convert it, which is going to fail in v19.06 due to a bug. That bug is fixed in the new version 19.06.15 available tomorrow. Or, today, with the current version, you can cancel the dialog and press + at the bottom of Playlist to add it. Clicking + will prompt you for a duration. It is not too important to set a correct duration because you can increase it when you add it to the Timeline by dragging the end of the clip.
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