Hi,
didn't mean to sound dismissive of the QGA. It is indeed narrow, but GM Bacrot (who plays both QGA and QGD) has a good piece of advice for practical play around QGA for Black:
- play 1.d4 d5 and of course, the QGA after 2.c4. [in his free blurb on modern-chess QGA repertoire for Black, Cheparinov notes that Black has no thoretical but also no practical problems to speak of. He goes for 3.e4 e5 which is now well studied, and notes that in the classical main line with 1.d4 d5 2.c4 dxc4 3.Nf3 Nf6 4.e3 e6 5.Bxc4 c5 (or ...Nc6 first, small differences) 6.0-0 Nc6 7.Nc3 a6, the popular 8.a4 is harmless (his words) and White should consider 8.d5 instead]
- against 1.d4 d5 without 2.c4, play 2...Bf5 and follow up with ...e6 and Nf6. For instance: 1.d4 d5 2.Bf4 Bf5 3.e3 e6 4.Nf3 Nf6 (holding back ...c5 on purpose) and now:
    a) if 5.c4 then 5...Bxb1 6.Qa4+ Qd7 (computers prefer
           6.Rxb1 Bb4+ 7.Ke2(forced) Bd6 but it's awkward)
           7.Qxd7 Nxd7 8.Exb1 Bb4+ Kd1 =
    b) if 5.Nbd2, then 5.Nbd7 (not allowing the reverse tactic
        5...c5 6.Bxb8)
    c) if 5.Bd3, maybe 5...Bb4+ 6.c4 Bxd3 and 7.cxb4 Bxb1
        8.Rxb1 c6 9.0-0 0-0 is possible. Doubled b-pawns don't
        help for minority attack.

Against 1.Nf3 Bacrot advises 1...d5 intending 2...Bf5 (except for 2.c4 d4), against 1.c4 he goes 1...e5 and either the Adhiban gambit 2.Nc3 Nc6 3.Nf3 e4 4.Ng5 c6!?, or development with Nc6, Nf6/Bc5/d6 against g3 systems.

So basically it's the QGA against 1.d4 d5 2.c4, and ...Bf5 systems against deviations not threatening an immediate Qb3. It is surprisingly efficient, e.g. 1.b3 d5 2.Bb2 Bf5 is totally OK for Black.

katar wrote on 05/26/23 at 00:35:01:

True, but Rublevsky usually answers 1.d4 d5 2.Nf3 Nf6 3.c4 with 3...c6, which is a different kettle of fish. Also 1.c4 e5 is fine for black, but I think 1.c4 c5 is more likely to appeal to the typical QGA player.

Black's choice after 1.d4 d5 2.Nf3 should be informed by what they do after 1.d4 d5 2.c4 dxc4 3.Nf3. There are actually a bunch of moves for black here: 3...Nf6, 3...c6, 3...e6, and 3...a6 are all important, and 3...c5 is possible. QGA players who want a main line used to answer 1.d4 d5 2.c4 dxc4 3.Nf3 with a careful 3...e6, which negates 2...Nf6 on account of 3.c4 dxc4 and black is move ordered. Black is equally move ordered if they would have chosen any of the "other" third moves besides 3...Nf6. Recently more popular after 1.d4 d5 2.c4 dxc4 3.Nf3 is a careful 3...Nf6 4.e3 e6 5.Bxc4 a6, and if this is black's intention there is nothing wrong with 2...Nf6. Also if black wants 1.d4 d5 2.c4 dxc4 3.Nf3 Nf6 4.e3 Bg4!? then only 2...Nf6 will do. Sometimes I play 2...e6 which I think is a reliable move. 3.c4 dxc4 is a QGA, 3.Bf4 Bd6 and 3.Bg5 Be7 are not the typical London/Torre white is expecting, and other moves black can play classically. Perhaps 3.e3 is a bit dry from black's point of view, therefore ... More often I play the ambitious 2...c5!? when white can pose problems only with 3.c4!. But weaker players never do this, usually it's 3.e3 cxd4!? 4.exd4 Nc6 with a type of Caro-Kann, else it's 3.c3 cxd4!? 4.cxd4 Nc6 with an Exchange Slav -- my favorite opening for black, but I'm strange that way.

After 1.c4 I have been known to "force" a QGA with 1...e6 2.d4 d5 3.Nc3 dxc4?! which Steinitz used occasionally but not many players since then -- for good reason. Not really recommendable but so far I have had good luck with it. 1...Nf6 is right out because of 2.d4 and black is move ordered, although I also play the Slav (and certain Caro-Kanns as mentioned) so I have further options of 1...c6 or 1...Nf6 2.d4 c6. White's threat of 2.d4(!) after 1.c4 really limits black's choices if they don't want to be move ordered out of a QGA, so it comes down to "preventing" the threat with 1...e5 or 1...c5, or having a second defense ready against the Queen's Gambit.

The great QGA proponent Rublevsky seems to favor 1.d4 d5 2.Nf3 Nf6 and 1.c4 e5.
I have another question for you.  What does a baseball player do at a dance recital?  Life is full of conundrums.

Essentially:
- If Black plays the KID, then Black can play Nf6, g6, Bg7, O-O, d6 against everything.
- If Black plays the QGD, then Black can play d5, e6, Nf6, Be7, O-O against everything.
- If Black plays the Slav ...Bf5, then Black can play d5, c6, Nf6, Bf5, e6 against everything.
- If Black plays the Slav Chebanenko (Chameleon), then Black can play d5, c6, Nf6, a6 against everything.
- If Black plays the Semi-Slav, then Black can play d5, c6, Nf6, e6 against everything.
Everything other than 1.e4. Though I mean technically Black could also play the same against 1.e4. Against 1.e4, the KID is essentially the Modern-Pirc, the QGD is essentially the French, the Slav ...Bf5 is essentially the Caro-Kann.
In some order. Like if White plays 1.c4, Black won't be able to immediately play 1...d5, Black will first have to play 1...e6 (QGD) or 1...c6 (Slav) and only then 2...d5.

But if Black plays the QGA, what does Black do when White doesn't play exactly 1.d4 d5 2.c4? What does Black do when White plays 1.c4 or 1.Nf3 or 1.d4 2.Nf3? Does Black play ...d5 and then whenever White plays c4 Black immediately answers ...dxc4? Would this transpose to lines of the QGA? What if White postpones playing c4 or never plays c4? What if White plays c4 but not d4?