Android Lollipop Not Supported?

I have a Kyocera Duraforce XD which i use for navigation because it is water, dust, shock- resistant.  However, it is maxed out at Android Lollipop (5.1.1).  I've used the Gaia app on it for the last several years.  But today the app disappeared and I think it may require Marshmallow (6.0) now.  Is that correct?  Is there any way to make the app work on older Android phones?  Would be a bummer since this phone, albeit old, works really well, is rugged and daylight readable.