The genetics of Germany and Low Countries is still a mystery to me. I do know there's diversity in Germany and Low Countries. "West Germans", "South Dutch", and Swiss, clearly have loads of Italian-like and or French-like ancestry. Austrians have loads of Polish-like and Yugoslavian-like ancestry. "East Germans" have loads of Polish-like ancestry. A simple Celtic+Germanic admixture doesn't work for all of them.
"North Dutch" and "North German" are similar to British/Irish and Scandinavians. They might be representative of the majority of Germans and Dutch, and have the most ancestry from Roman-era Germania. They have loads of Y DNA I1(like 20%) and R1b-U106(like 20%). However, they have hardly any R1a-Z284, while Scandinavians have around 20%. That's a hole in the theory, hordes of Scandinavians in the Iron age brought Germanic languages to Germany.
There's lots of possibilities. Germans definitely aren't purely from Bell Beaker and Corded Ware. The 1200 BC German guy who had R1a-Z283, clearly isn't the same as modern Germans. Their ancestors were apart of the same North European Steppe+EEF family, but aren't sampled yet by ancient DNA.
I tend to think, irrespective where Germanic languages began(It's either Germany/Denmark or Scandinavia), modern Germans IMO are a mixture of "Germania" and other(French-like/Gaul, Italian, Polish, Balkan).
Gauls are the most famous Celts, because they're the Celts Romans knew. That doesn't mean Gaul was the center of the Celtic world. But actually there was never Celtic world that extended to the British Isles. No one, as far as we know, was aware that the languages in the British isles were apart of the same language family as the languages in Gaul. People in the Isles and Gauls weren't considered to be the same people. So, there was no concept of a pan-Celtic world. There was a concept of Gaul and Britain and Ireland.
Celtic languages could have expanded with Bell beaker culture to the Isles. We can't dismiss that possibility, and state it a fact that Celts of the Isles are descended of pre-Celtic Beaker folk. If that's true, it'd be difficult to measure Celtic vs pre-Celtic ancestry, because the people who brought Celtic languages would have been pretty similar genetically.