Healthy weight gaining foods:
R e d m e a t s: 
P o u l t r y: 
O i l y f i s h : 
C h e e s e: 
D r y Fo o d s:
W h o l e e g g s
Unhealthy weight gaining foods (Eat it's with moderation):
Pizza
Fried foods
Burgers
Hot Dogs
(goes on)
Here you go.
Hope this helps!