50s-60s is too cold for this species, even more so when potted in S/H, which further cools the root system. Ideally they like temps in the high 80s to low 90s, but grow fairly well at lower temps too (normal intermediate house temps), though will slow their growth. That being said, the plant looks overall quite good, that yellowing leaf could be normal leaf drop, but it's hard to tell from one angle.
Here are some specifics about this species that are good to know, and which you can think about in relation to your plants problems.
- It hates having its roots disturbed, and is known to sulk after repotting. A large, healthy root system is important for this species since it must support many large leaves on an older plant.
- It is prone to crown rot due to its leaves growing tightly together, so should never be watered from above or get water between the leaves. This is why it's most often grown mounted.
- Even more than other Phal species, this species will end up growing sideways if grown potted, and hang over the side of the pot. Trying to keep it upright is a losing battle.
*Unlike most Phals, this species needs high light. In the wild it grows high up in the canopy, so takes Cattleya level light with good air movement.
It's hard to say anything about the stalled growth being light related, that depends on the output on the lights, duration they are on and distance to the plants. But if your other Phals are fine under those lights, then it's probably not enough light for Phal gigantea. I suspect that the low temperature it had until recently also has to do with it.
Phal gigantea
. Hopefully the warmer temps will perk him up and next winter I'll be sure to keep him warm. 
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