Just how fast is this "very fast gain"?
If 6# literally piled on overnight, chances are it isn't a gain at all, just a normal fluctuation in water retention. It is very difficult to attribute blame for a "gain" that comes on fast....
If it's been piling on a half pound here, a half pound there, with the scale trending up over a period of weeks, then yeah, you've probably got a legit gain going on. But the reason for it is likely to be "too many calories in vs calories expended." You can overeat on ANY eating regimine, low fat, low carb, vegetarian, gluten-free - any of these have the potential to be helpful OR harmful to our efforts.
Are you weighing and measuring your portions? Do you know how many calories you are taking in each day? Is it more or less than your Spark suggested range?
Are you unhappy and/or bored with your low-carb choices, which is setting you up for periods of "not wanting to do it today" or "cheating" or "binging" or other over-indulgences?
If you don't like "low carb" - pick foods that you do like. And then implement regular weighing/measuring/journalling and exercise good portion control. If you do this, it won't matter if you're low-carbing, high-carbing, paleoing, or whatever.
Goal 1 - break 200 (46 pounds lost)**DONE**
Goal 2 - leave obesity behind (BMI 29.9, at 185#) **DONE**
Goal 3 - BMI = Normal (154# or less)
| current weight: 172.5