Which countries would you consider to be Germanic

[Domias]

I think France,
Switzerland and Benelux are genetically mostly Celts but Latinized or Germanized, depending of regions, except Brittony which stays Celtic.
The ancient Gaul was invaded by a lot of tribes (especially Germanic ones) but the influence was more cultural than genetic because they didn't represent an important number of persons compare to the Gaulish population . So I don't consider them as pure Germans (guys, please don't think it's a negative opinion)

[tteo]

France could only be considered Germanic if we were to be talking about specifically of the Normandy people.

[thatoneton]

For me the ones which I see as "solely" Germanic are Scandinavian ones (Sweden, Norway and Denmark, some small Finnic influence in Sweden notwithstanding). Iceland and the Faroe Islands, though being islands in the middle of the Atlantic, can count as well as they were settled by the population coming from modern Norway).
The Netherlands (Frisia, Holland etc.) surely count as Germanic.

As for the rest of those typically considered Germanic - most indeed are, but with a "but":

*Austria - half Germanic, half Slavic (Austrians come from Bavarians who expanded into the Carinthian lands in Central Europe [Carinthians are the ancestors of modern Slovenians])

*Germany - 2/3 Germanic, 1/3 Slavic (despite being probably the most obvious choice for a typical Germanic country modern Germany contains a significant amount of lands formerly not inhabited by the Germanic but rather the Lechitic (Polish) tribes and peoples in what is now East Germany and additionally it formerly held a large swaths of land in Central Europe (Poland and Czechia) and the Baltics, with Germanization of the local population contributing a significant amount of admixture of West Slavic genes. To this day many major cities in modern East Germany, like Berlin or Lubeck holds the names preserving the language of its former inhabitants.
To this day around 3 million Poles (mostly later immigrants) are the biggest ethnic minority in Germany, with another closely related group descended from the Lechitic tribes, the Lusatian Sorbs, still existing close to the border of Poland and Czechia.

Switzerland - actually surprisingly Germanic given its rather mixed population. A Calvinsist faith makes them both somewhat distinct and also pretty Germanic, however it still obviously is not even close to being fully Germanic.

England - modern English people are not necessarily closer to the Germanic people than they are to the Celts from the British Isles and their language is a mix of Romanic (Latin and French) and Germanic (Anglic).

Belgium - a curious case of a state with two major ethnic groups, each of which feels closer to the neighboring country rather than its own.

[Flamingo]

All countries and regions in Europe where a Germanic language is spoken.

[earthtube]

I could be incorrect on this but as far as I understand, only a small portion of East/Southeast England traces the majority of its genetic material to a Germanic source, though I think it should be pointed out that regardless of the specific ethnic composition of the people, they still speak a predominantly Germanic language, with some of their myths and customs being derived from Germanic rather than Celtic/Roman heritage. Of course a significant amount of English vocabulary and grammatical structure are derived from Norman French, Latin, and to a lesser extent Brythonic. In my mind, Great Britain is a unique melding of Germanic and Celtic/Roman, with predominant political influence from either group passing back and forth across the centuries until the present day where most of the population of Britain (save for remote parts of Wales) have not-insignificant Germanic Anglo-Saxon admixture with their Romano-British Celtic heritage. I find the assertions by the British aristocracy that their heritage is predominantly Anglo-Saxon to be a bit dubious given the historical context. When the Norman French aristocracy took power in 1066, much of the traditional Anglo-Saxon aristocracy was liquidated and replaced by Norman French and Celtic Breton nobility. This was well understood at the time, and many of the Celtic nations retained the cultural memory of at one time being politically dominant on the island. This is suggested by the Prophecies of Merlin in Monmouth's Historia Regium Britanniae, and the Celtic nations may have seen the Norman conquest as a restoration of the balance of power between the two. It should be noted that the not-so-subtly anti-Anglo Arthurian legends flourished from 1066 through the Tudors, a period of the political and cultural ascendance of an aristocracy aligned with sovereigns of Celtic and Roman heritage (though the Germanic Anglo-Saxon heritage was always present as well). England's cultural Renaissance was under the proudly Welsh Tudors, and they were succeeded by the proudly Scottish Stuarts. It was not until the German House of Hanover took power in 1714 that British royalty and aristocracy decided to re-assert and attach themselves to a strongly Germanic heritage, which in a sense was a re-writing of history to align themselves with the newly-ascendant Hanovers. It makes it complicated to define Great Britain along an axiom of German-ness, and I think reductive. Great Britain is British, and I venture that after nearly a millennia and a half of cultural/political development and population intermarriage the vast majority of British people are neither Germanic nor Celtic, they are something new.