Disadvantageous Alternate Form is a disadvantage version of the Alternate Form advantage. It was explained in the designer's notes for GURPS Powers by Sean Punch at

a disadvantage, worth -5, -10, -15, or -20 points for Rare, Occasional, Common, or Very Common trigger conditions, respectively.
The point value of the form isn't important . . . unless the GM feels it is. Turning into something extremely vulnerable, like a clam, can be a problem. Becoming the Death God can be useful if you abuse it properly; just be sure you're on enemy territory when the trigger gets pulled.
As such, the GM might wish to add a percentage of the point-value difference between the shapeshifter's base template and the shifted template to the above cost -- perhaps 5%, 10%, 15%, or 20% depending on rarity; perhaps more.