Hi, Marc. If you're looking to identify your grandmother's father, a yDNA test won't offer you any help. I'm a yDNA testing fan in general--so if you want to grab a test right now while Family Tree DNA is having a sale--I say go for it if you'd like to add that arrow to your quiver of research tools.
But that great-grandfather didn't pass along any Y-chromosomal information to your grandmother, who of course had none to pass along to Helen. Likewise, mtDNA won't help in this scenario, either: the unknown grandfather contributed no mitochondrial DNA to his daughter.
For DNA research on this line, autosomal DNA is the only choice. What you can do with your 23andMe raw data is dependent upon when you were tested. If under the newer GSA chipset--this would be 23andMe v5--you can download your raw data, and then upload it to a free account at GEDmatch Genesis. If it was a 23andMe v3 or v4 test, you have the option of using the results on the regular (non-beta) edition of GEDmatch, plus you can upload to Family Tree DNA (and My Heritage, if you choose) for a wider reach of possible matches. FTDNA charges a $19 fee to "activate" the upload so that the results can used in locating matches. I don't yet have any experience with My Heritage, but a friend convinced me that I needed to get familiar with it; on the to-do list next week. :-)