As far as I understand, attaching an L-function to an automorphic representation attached to a general reductive group G is conjectural and still open.
The way one attaches L-function depends on a representation r of LG and partitioning the set of irreducible admissible representation of G(kv) in to L-packets (which is conjectural in general and known in very few cases). Assuming one can define local L-packets, Borel and Tate's article in Corvalis explains how to attach L-function to it. But still this L-function depends on the chosen representation r.
If pi is an irreducible admissible representation of GA then pi=otimesvpiv, where piv is an irreducible admissible representation of G(kv). So assuming we can partition the set of irreducible admissible representation of G(kv) in to L-packets, piv belongs to L-packet Piphiv corresponding to some admissible homomorphism phiv of Weil-Deligne group to LG/kv . The repesenation r defines a representation rv of LG/kv.
Then the L-function attached to pi and r is defined as:
L(s,pi,r)=prodvL(s,piv,rv),
L(s,piv,rv)=L(s,rvcircphiv)
Now rvcircphiv is a represenatation Wiel-Deilgne group, so by Tate's article in Corvalis, we know local L-factor.
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