Assuming you mean weight (resistance) training when you say “Gym”, you're actually comparing two somewhat different, but, complimentary activities. So, if at all possible, why not do both?
As Alec states, your worry about a “gym body” is unfounded for the reasons he lists. However, that should not discourage you from participating in an exercise program to add/improve muscle mass since studies have shown that there are numerous advantages to building a base of muscle. Also keep in mind that muscle is metabolically active, thus assisting in the burning of unwanted calories. In fact, a recent study found Resting Metabolic Rate (RMR) was indeed lower for those individuals with lower amounts of mass:
As expected, the RMR of women was lower than that for men
(4,12,13,19,27), and the RMR of older adults was less than that for
younger adults (9,13,22). Some of the differences between the sexes
and age groupings could be related to muscle mass being lower (e.g.,
less metabolically active tissue) in women and in older adults. RMR is
mostly dependent on the amount of metabolically active tissue in an
individual; mainly muscle mass (18,35).
(Examining Variations of Resting Metabolic Rate of Adults: A Public Health Perspective
Bicycling, as a complimentary activity, can assist in further burning calories, as well as, providing the many benefits that can be achieved from cardio exercise. Many gyms offer various cardio equipment including bicycles.
In the end, whatever path you decide, your weight loss success will be directly related to consistent work, sound nutrition (as Alec reminds us), and a long term commitment to exercise.