The brindle leads us to pitbull terrier, plott hound, and great dane -- I agree with Molly.
The ears (the gsd was crossed with something longer eared) -- that doesn't rule out any of the above three. But it does rule out Dutch shep.
Geography may rule out Plott -- are they uncommon in your area?
If it's an urban humane society, that raised the probability of pit.
As Molly says, what size is it? And how old? If under 7 months, the ears may still go up!
Alternatively: the brindle and shep side could be Dutch shepherd. Then the cross could be with MANY BREEDS.
Read an article where a lady with a purebred am-staff, black/white spotted - generations of am-staffs in his pedigree - did the dna "mixed breed id" test - and it came back border collie, jack russell terrier and pomeranian or some such thing. So no, I wouldn't trust that dna test.
Catahoula come in the leopard color, black and white, black and brown with a saddle like a hound, hound like spotted, brindle, solid colors... many variations in color. They are a breed who are bred to work,not for color. May they never become popular! They are extremely versatile. AKC now trying to gather pedigrees...grrr. Not into designed dogs, but a Cat Shepherd mix, I would entertain at home.