Looking over the program, I think that with the gains you've made, it's time to move to a more standardized training protocol if you want to keep moving forward.
Circuit training is fine for a few months when first getting into weight training, but it has drawbacks as well that show over time. First, you're typically always moving and not getting rest between sets - that's great for building endurance, but can hold back strength and size over time. Not to mention that it leaves out some things that are the best for packing on weight, typically you'd want to put more emphasis on heavy compound lower body work with squats (which I see as a "warm up" lift only) and regular deadlifts. And, there comes a point for many people where hitting the same body parts multiple times in a week slows down recovery, so you may be better off with more time between hitting the same parts unlike how it is with the program you noted. Also, am I correct in it just being 2 circuits as well? If so, chances are that you're now experieneced enough that you may require more volume in your training protocol to really move things to the next level for gains.
I would recommend that, if you can do a 3 day per week split, you might want to try something like this:
Sunday - Legs and Chest/Triceps training
1. Squats - 2-3 warm-up sets, 4x8 (4 sets of 8 reps) afterward, coming as close to failure on the last rep of the last set (failure is not always desirable, but you want to be close). I usually go with the feel of first working set I could probably hit 2-3 more reps if I fought for it. 2nd set, maybe 1-2 more with a lot of work. 3rd set, not likely I could do any more reps beyond the set number, last set to where it takes everything out of me to get through it, no possibility of getting more reps. For rest times, I'd suggest two options - either take 2 to 3 minutes between sets, or, take 1 minute, move to do a set for chest, rest one minute, and rotate back to leg work again (same for other days, either take a moderate rest period if sticking in one lift or take 60-90 seconds, move to other body part, and alternate between). Just be sure that, as you're moving into heavier compound lifts to give yourself enough time to recover a bit between sets, otherwise, you might not gain as well if you keep rest periods too short when hitting the same part.
2. Barbell or dumbbell bench press, 2-3 warm-up sets, 4x8 afterward, same "feel" as with squats for how tough sets should be
3. Leg curls (using either seated leg curl or lying leg curl machine), 3x10-12 reps, no warm-ups sets
4. Close-grip (hands at torso width, not too close together) barbell bench press, either flat or with slight incline, 3x10-12 reps, no warm-up sets
That would be enough for that day, unless you feel your calves are lagging and want to throw in 3 sets of calf raises, but that would be totally optional
Monday - rest or light cardio work if desired (30 minutes low to moderate intensity maximum)
Tuesday -Upper back, biceps and ab work (if needed)
1. Barbell or dumbbell rows, 2-3 warm-up sets, 4x8 for working sets, same feel as described above
2. Pull-ups (if you can do them) or pulldowns, 3x10 for working sets, no warm up needed
3. Barbell or dumbbell shrugs, 1-2 warm-up sets, 3x12 for working sets
4. Cable face pulls (check Youtube for video, just need a rope handle and normal cable tower setup at gym), 3x15, no warm-up needed
5. Optional biceps work done as dumbbell hammer curls, 1-2 warm-up sets, 3x8-10 for working sets, only if you feel the need to do additional arm work
6. Optional ab work if desired, 3 sets of whatever you enjoy doing
Wednesday - rest
Thursday - Lower back and shoulder work
1. Deadlifts from the floor - 3-4 progressive warm-up sets, then 4x8 working sets
2. Standing barbell overhead press - 2-3 warm-up sets, 4x8 working sets, bar touching collarbone in front of head each rep, don't cut them short at ear level, go all the way down
3. Single leg dumbbell Romanian Deadlifts (check Youtube for clips of how to perform), 3x15-20 sets
4. Standing 1-arm dumbbell clean and press - 3x10-12, slight lean to the side allowed for stability
Friday and Saturday - rest, or light cardio
Pretty simple program, working non-interfering body parts with enough time for most to have plenty of recovery, utilizing good compound lifts as the base to provide the most bang for your buck. Also gives off the two main days people usually like to go out and have fun, so you don't have to worry about training for the main part of the weekend if you don't want to (if you prefer, just move the days forward by one or two in the schedule but keep same pattern of days training/days off). Try something like this for 6-8 weeks, add a bit of weight each week that you're able, see what happens!
"A 'hardgainer' is merely someone who hasn't bothered to try enough different training methods to learn what is actually right for their own damned body." - anonymous