Coincidently, I, too, am going through the Gielen cycle and thoroughly enjoying it. In fact, it might become my favorite modern Mahler cycle if things continue to go this well.
Interesting choices for each symphony, I think I’ll play, too, so here’s mine:
1st - Kubelik/Symphonie-Orchester des Bayerischen Rundfunks (DG)
2nd - Solti/LSO (Decca)
3rd - Bernstein/New York PO (Columbia/Sony)
4th - Abbado/Wiener Philharmoniker (DG)
5th - Bernstein/Wiener Philharmoniker (DG)
6th - Tennstedt/LPO (LPO house label)
7th - Rattle/CBSO (EMI/Warner)
8th - Boulez/Staatskapelle Dresden (DG)
9th - Haitink/Concertgebouw (Philips/Decca)
DLvdE - Jurowski/Rundfunk-Sinfonieorchester Berlin (Pentatone)
Some of these choices
may surprise some as I’ve been known to spout off about Mahler on this forum from time to time.

I didn’t include the 10th, because 1. I don’t consider it’s completed form by Deryck Cooke to be too interesting, 2. for me, it’s just not a part of Mahler cannon and 3. he never finished it and only left the
Adagio, which I do like a lot, but I don’t consider it a part of the official symphony cycle. This may make me an “outsider” to popular opinion or people’s opinion on this forum, but I don’t care. I’ve tried to love the completed 10th for 15 years now and it just has never worked for me as a substantial piece of music that stands on its own.