My comment will be based purely on linguistics, so this may not be what you are looking for, it still answers your question in a certain way, but not fully.

Both Celtic and Germanic languages are part of Indo-European languages, like most other languages in Europe are. Inside the Indo-European family, different language subdivisions hold different proximity to eachother. For example Baltic and Slavic languages are closer to each other than for example Germanic and Celtic, which is deducted by similarities between the languages, and archeological evidence where possible. By following different languages through time, and looking at new inventions, we find out approximately when a language group split off into two. According to Chang, Will; Chundra, Cathcart (2015) article, Germanic languages split of from Italo-Celtic first, followed later by the split of Italo-Celtic a few centuries later. Another article from 2013 groups Balto-Slavic as the closest group to Germanic, and Italo-Celtic as the second closest. (Also do note that Italo-Celtic has a history of going in and out of favour with linguists and there is no consensus of its existence). I haven't seen a modern study that would link Celtic and Germanic as the closest relatives of each other, and none that would put Germanic as a "descendant" of Celtic, as consensus is, that Germanic languages emerged before Celtic ones.

So to answer your question: no, Germanic people are not descendents of Celtic people, atleast not linguistically. There might be generic evidence that show a different picture (Celts assimilating into Germanic), or there might be archeological evidence that show relation between the people that would explain the possible incorporation of Celtic gods into Germanic pantheon, be it with trade, war or some other way. There are many cases in history where foreign gods got incorporated into new religions, so that might be the answer you are looking for.