Hi,
Welcome to Healthcare Magic Forum.
I have gone through your Query & I really appreciate your concern about the problem you are facing.
As a physician I would like to tell you that there is no direct contributory side effect of 
Losartan as weight gain. But, there are other contributory side effects like 
anxiety, increased hunger which may lead to ultimately increase in weight.
These are some common side effects of Losartan :
    Abdominal or stomach pain
    anxiety
    bladder pain
    bloody or cloudy urine
    blurred vision
    chills
    cold sweats
    coma
    confusion
    cool, pale skin
    depression
    difficult breathing
    difficult, burning, or painful urination
    dizziness
    fast heartbeat
    frequent urge to urinate
    headache
    increased hunger
    irregular heartbeat
    lower back or side pain    
nausea or vomiting
    nightmares    
numbness or 
tingling in the hands, feet, or lips
    pale skin
    seizures
    shakiness
    shortness of breath
    slurred speech
    troubled breathing with exertion
    unusual bleeding or bruising
    unusual 
tiredness or weakness
    weakness or heaviness of the legs
Thus, I should advise that you should visit your consulting physician to get your doubts cleared or get shifted on another variety of a 
anti-hypertensive medication.
Thanks for Trusting & Posting your Query on this forum.
Hope this will help you. Feel free to ask any further question.
Have a nice Day.
Regards,
Dr.Abhijeet