The G550 PCI needed one of those cables, the AGP version just had DVI-I and VGA on the bracket so 'just worked'.
Now, as for the question of how good it is for retro gaming, the question is what 'gaming' and how 'retro' exactly.
The G550 was never designed for gaming and although it supports D3D and OpenGL and offers EMBM, its performance is hardly any better than Matrox' 1999 G400 cards (slightly surpassing the G400Max in OpenGL but lagging behind the regular G400 in DirectX 6). The G400 in turn was comparable to an nVidia TNT2, a G400Max to a TNT2 Ultra (in DirectX; in OpenGL the nVidia chips were significantly faster - Matrox always struggled with their OpenGL drivers), so despite being a 2001 card, this is late 1999 high-end performance.
So, is late 1999 high-end performance enough for the games you want to play? Doom 2 is a 1994 game so no problem in terms of performance, it's so old that Matrox' poor SVGA VESA support didn't matter. Q3A and HL1 will run happily with it. HL2 is a 2004 DirectX 8.1 game. No way will that run smoothly or indeed even looking as intended on a G550. A Matrox Parhelia could at least run it looking the way it should, but to play HL2 well needs a GF4 or Radeon 9800 or faster - and comparable 2004-era CPU.