if you want to gain weight you must consume more calories than you burn. the guys in the gym that get big fast eat a ton! the also do a lot of supplements. but what they eat, the real serious guys, is very healthy. they have done their homework. now you can eat a lot of junk and put weight on fast, but it will be fat, which i assume you dont want to put on. you want healthy weight - muscle. this is a slow tedious process. i was down to 130 pounds about a year ago. i've managed to put on close to 20 pounds since then. i did it with weight training, surfing, and tai chi. but i didnt eat junk. i ate lean meat, fruits, vegetables, minimal grain based foods, and NO processed foods/sugars/grains etc. i ate REAL food with an occasional protein shake but only after an hour or so after gym workouts.
for a lot of good advice on this subject, check out t-nation.com. lots of good info on how to train and what to eat. you may have to adapt the diet a bit to fit your individual needs and stay away from your danger foods though. however i have seen posts there from individuals who have crohn's disease on how to train and what to eat, so dont be afraid to ask and think if you do the results will be fruitless. just be careful of the supplements any one there recommends. imho, most dont do anything, with others need you to stick to a pretty strict regiment which is difficult for us with ibd, and still some individual results vary. but i think that sight can help. my brother recommended it to me for the same reason i am recommending it to you. he's become my pseudo personal trainer. i'm still lean, but the 20 lbs i put on is solid muscle, and that takes time to do so you must be patient.
best of luck!
Crohn's since 1993 (17 yrs old then)
surgery in July '05 - removal of 2 inches at ileum and 8 inches of sigmoid colon (had fistula into bladder)
Nov '05 developed colonic inertia; July '06 told i needed ostomy surgery
began maker's diet in August '06 - now feeling the best ever with no symptoms of colonic inertia and i kept my colon
med free as of 10/31/07