These gangers look ace! Definitely a blanchitsu look to them. Can I ask where you've got the torsos/legs from?
In terms of critique/improvement, do the added armour plates on the first two gangers have any in-world viable support? The loin plates might have a belt behind them but at this angle, the first ganger's shin plate/greave looks like it's held on by art. Raggedy or crude straps also help to add extra textures, which to me is also a hallmark of blanchitsu style. A bit of greenstuff with perhaps a couple of nicks/notches along the edges will replicate worn leather or thick fabric relatively easily.
As for greenstuff tips,
@undertaker is bang on with the colour shaper advice. Personally I went with the extra firm version (black tips) that are generally called clay shapers and a set of five 0-size for the different shapes - the cup chisel (see below) is the only one that doesn't see regular use.
Flat and cup round are great for smoothing, angle is good for sharp creases and narrow dents, while I find the taper ideal for doing smooth folds in cloaks and fabrics. If you just start with one however, flat is definitely the most versatile.
I still wet my shapers with water (or my tongue if I'm not thinking) while working freshly mixed greenstuff. A final smoothing pass once the greenstuff is almost too hard to work -round about an hour after mixing- is always a good move in my opinion, as ridges can be smoothed down without the putty moving very far and a silicon tool like a shaper won't stick at that stage either.
If you do find cracks/dimples/rough spots after the greenstuff is cured, milliput is a useful putty to have around. Water will dilute it into a paste that can be smeared into imperfections and it will take a sanding much better than greenstuff if you need to go that far. Just make certain that tools are cleaned off before the milliput cures, because it will bind to practically anything with the determination of a face-hugger!
I like the yellow-grey milliput for fixing jobs; not only is it the cheapest variety that I know of, but it's pale enough that if applied after priming it will still cover easily with a brush-on primer, rather than re-coating the whole model.