Old English does not have a word for 'gas'. So I have now ruled out two methods of vocabulary-sourcing. I looked at the English Wordbook (a dictionary for a form of restrained English named Anglish) and found the listed alternatives for 'gas' to be 'wind' and 'loft'.
These would yield 'wind' and 'lyft' in my West Germanic. However, they already mean 'wind' and 'air'. I think the distinction between 'gas' and 'wind' is a necessary one.
To maintain the distinction, I have two options. I could forget Anglish, go all the way back to the Proto-Indo-European root of 'gas' and find a word in Old English from which it evolved. This is tricky. The alternative is to adopt phonemic tone, or what is known as a pitch accent. It already exists in the majority of the world's languages and in North Germanic ones, such as Swedish, too. Not many of my words will require tone, but I can think of some that would benefit. I wanted to use 'erdappel' (earth apple) for 'potato' based on similar words in other languages. However, 'eorþæppel' (yielding 'erdappel' in West Germanic) is a native Old English word for 'cucumber'. And I need that word.
With tone, I could give priority to the native meaning, yet add an additional meaning.
wind - wind
wînd - gas
erdappel - cucumber
êrdappel - potato
I have not yet decided what tone I should use.