"Dafür biete ich mich gerne an."

[retsaylor]

can gern be spelled gern and gerne?

[Hohenems]

Yes it can. From what I understand through native speakers that contribute to Duo, there is no reasoning for one over the other, it is just personal preference.
http://en.pons.eu/dict/search/results/?q=gerne&l=deen&in=&lf=de

[amyb24]

What is the role of Dafür in this sentence?