I have never read an article where DIM had an effect on "gains" is you are talking about hypertrophy.
On the other hand I have seen articles where DIM was linked to prostate health and reproductive health (mostly in women). It is also seen as an antioxidant and estrogen balancer. I say balancer instead of reducer as estrogen is a vital hormone in both men and women so the idea isn't to eliminate it, rather you want the optimum ratio of estrogen to testosterone. Something that is seemingly difficult now a days due to the xenoestrogen boogy-man lurking within every grocery product.
Last time I looked into DIM the standard dose was between 100-200mg, 1 or 2 times per day. Basically it will boil down to about $10 a month to supplement with it and it is seen as "safe" unless you are taking other prescription drugs, or pregnant, or nursing.
And as with all my advice; don't take my word for it ask your doctor if said product is right for you