Spread graham cracker crumbs on bottom of 9 in. (23 cm) baking dish and press down to tighten crumbs; refrigerate.
Dissolve lemon gelatin in boiling water and add lemon juice and sugar replacement.
Chill mixture until congealed.
Beat until fluffy.
Fold lemon mixture and nuts into topping.
Pour mixture into baking dish; refrigerate 3 hours or overnight.