> og báðu hann setja aðra ráðagerð þá er Gunnari væri til meins.

Alan: and asked him to set (devise) another plan when (it would) be to
(the) harm of Gunnar.

Grace: and told him to make another plan then which was harmful to Gunnar.

I think 'þá' is more likely to be the fem.acc.sg. pronoun here,
agreeing with 'aðra ráðagerð': "...another plan *which* would be of
harm to Gunnar." When a pronoun is used like this before the simple
relative 'er', it takes the case appropriate to the main clause.
Here's an example from Heimskringla [
http://www.snerpa.is/net/snorri/okyrra.htm ] where the combination
'þá...er' is used in both senses, "which" and "when", in the same
sentence:

En er hann var austur í Ranríki á Haukbæ að búi sínu þá tók hann sótt
þá er hann leiddi til bana.
"And when he was in the east, at his estate at Haukboer in Ranriki, he
went down with the illness which led him to [his] death."

In this example, the first 'þá' is "then" and correlates with 'er'
near the beginning of the sentence; the second 'þá' is the fem.acc.sg.
pronoun that refers to 'sótt' and goes with the relative particle 'er'
immediately after it.

LN