If you look at the Ancestry match list, you can do a search for the name Yost and you will see matches with that surname. Look at their trees and see if their Yosts are from Germany or Netherlands/Belgium. The spelling might be different, and you will see that in your match list.
Right, if you are not showing German DNA, it is possible that that Gerhardt is from somewhere else. That will be apparent as you look at your match list, and don't see any German matches.
With German great grandparents you should be showing about 12% German DNA. If instead you are showing 12% Slavic DNA, that might be the answer.
You can also look for Scot, Danish and Swedish matches.
I know for a fact that 23me is much better than Ancestry in picking up small amounts of Native American DNA. So for sure you should test at 23me. FTDNA and Ancestry will not pick up small amounts of NA DNA.
The NA DNA would have to come from your direct ancestors. So one of the two brothers and his wife will have to be direct line ancestors.
You also might want to make a family tree, to see if you can find the two brothers marrying NA and how they are related to you. Sometimes family stories are accurate, some times not. The DNA will tell you the truth.
There is also a free site called Gedmatch which is very good at picking up NA DNA. It doesn't sell kits, you just upload your DNA results from Ancestry or 23me for free, and then there are good tools you can use to analyze your DNA in sophisticated ways. You could also see if it picks up German DNA.
If the current relative who tested is a cousin, I would recommend you also test at Ancestry if budget permits. If it is a sibling then you could skip that.
Let me know if I can help in any way. This is a hobby of mine and I love doing it.
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