Confining ourselves just to the TSP system, I think the actual facts are more subtle.
The only way into the TSP is regular small deductions every paycheck, which looks like DCA. It is DCA if you pre-allocate that deduction to some mix of F, C, S, or I, and never or rarely re-allocate once you buy. But if your contributions are 100%G, you are essentially accumulating cash in anticipation of a lump sum trade into one of the funds whose price can actually go down. I guess you could do an even more extreme lump sum strategy by setting your contributions to 100% of your pay to G for 3 months, then 0% for 9, but I doubt if many do that.
So, 100%G is a lump sum strategy, and 0%G is DCA.
Personally, I now do the former, but did the latter for 20 years. For most of those years, you couldn't trade daily, and your choices were stocks, bonds or cash, so the ONLY way to allocate was to DCA upfront and fix things during "open season." I DCA'ed into the C fund.
Now we can day trade. Buying 100% G sets up lump sum investing, and things get interesting.
Once in, though, who DCAs inside TSP? Nobody. Of the people who are tracked on this site, everyone swings in big trades: Tom just went 100%G in one trade. Others might go from 50%I to 10% I or whatever, but they do it all in one large trade.
DCA'ing inside TSP would mean going from 50% to 10%I in a series of, say, 20 2% trades. In that sense, almost nobody DCA's. Single large trades.
Therefore, for TSPer's DCA means differential allocations at the contribution stage. If we wanted to compare strategies using real data, we'd need a tracker that, say, only moved between C and I, and a tracker that DCA'ed into the C and I initially but didn't trade thereafter. For a fair comparison, their long-term averages would have be the same, e.g. 50C:50I.
So the comparison would be: 100%G contributions followed by sporadic, random (or non-random, "timed") buys of 50C:50I, versus initial "DCA" 50C:50I buys. The financial gurus say none of it matters. In the long run, random and DCA don't differ. The only one with a chance to beat the averages is the non-random timed buy, but beating the averages is very, very difficult.
Jonathan