I think I'd stay away from any gift that put a price tag on it (ie, gift cards). It was a gift from the heart. A note is likely enough. A baked good or food basket, icing on the cake.
I love gift cards in general, but this time it feels a bit different to me, for some reason...Or, I guess, as I write this, that a gift card for gas is sort of a cute idea above. But, I wouldn't do just a generic gift card, I don't think...(not, if I were in their shoes, that I would in any way judge anything you did or nothing at all!)
me:smc (single mom by choice)
Dd: Conceived when I was 42 after 2 years ttc. Conceived on 6th IVF cycle after 2 bfn's and 3 m/cs.