From TALES OF THE UNEXPECTED 219, Feb 1982:
"Dust Devil" 5 pages, by Gary Cohn and Keith Giffen
https://comiconlinefree.com/tales-of-the-unexpected-1956/issue-219/31

It's the only story I can recall where Giffen did both pencils and inks. A Great Depression-era town ponders a deal with the devil to get themselves out of economic ruin.

Giffen left DC abruptly in 1978, and Levitz was kind of pissed about Giffen leaving without notice on ALL-STAR COMICS 60-63.
https://comiconlinefree.com/all-star-comics-1940/issue-60

After several years, he came back to DC in late 1981, and had to prove himself reliable before he was given a regular series, initially just given shorter backup stories in late 1981-early 1982 in this story and GHOSTS 104, 106 and 111:
https://comiconlinefree.com/ghosts/issue-104/26
https://comiconlinefree.com/ghosts/issue-106
https://comiconlinefree.com/ghosts/issue-111/14

and HOUSE OF MYSTERY 284 and 301:
https://readcomiconline.to/Comic/House-of-Mystery-1951/Issue-284?id=70834#14
https://readcomiconline.to/Comic/House-of-Mystery-1951/Issue-301?id=70858#11

and in UNEXPECTED 219 and 222.
https://comiconlinefree.com/tales-of-the-unexpected-1956/issue-219/31
https://comiconlinefree.com/tales-of-the-unexpected-1956/issue-222/25

These shorter one-shots offer a glimpse of Giffen's work on a wide range of stories and genres, with a wide range of inkers, and DC apparently settled on Mahlstedt as the best choice for inking Giffen.

Right after, Giffen was given series work on LEGION 285-306:
https://comiconlinefree.com/legion-of-super-heroes-1980/issue-285/full
And actually, the first two LEGION issues Giffen did were backups too, before DC gave him the lead feature with issue 287.

And on a Dr Fate backup series in FLASH 306-313

Both of which I think present Giffen's very best work.