My understanding is that these people were celts.
That the people of britian were not actually genetic celts but rather partook of the celtic culture that spread through europe after their arrival in europe around 1200 BC. For example the welch and the irish were more closely related to the coastal peoples of spain and portugal.
The Helvetii, who were later replaced by the Germanic tribes, were indeed Celtic, and were conquered and/or coopted by the Romans. They were an earlier coat of paint, as it were.
Ancient genomes reveal social and genetic structure of Late Neolithic Switzerland
https://www.nature.com/articles/s41467-020-15560-x