Am I allowed to give a gift to my wife while she's an aval ?
After the shloishem?
Answer:
Yes, after shloshim. The gift should be something not so significant and therefore does not cause excessive happiness.
Sources:
The Mishnah Berurah (ibid. 21) quotes this ruling, though he brings it from Magen Avraham (who explains differently), which makes his opinion a little unclear (seeDivrei Sofrim, aveilus, Eimek Davar 47); he adds, however, that the gift should not be something that causes simchah. The same ruling is given by Derech Chaim.
Therefore, under circumstances where the gift is “required” as part of a relationship, for instance from husband to wife, there is room for leniency (see a similar ruling in Nitei Gavriel, Aveilus Vol. 2, Chap. 14, no. 13).