Can you turn an old laptop into a gaming pc?

Doubtful. I retired a desktop PC of similar vintage last year after the networking adaptor screwed-up.

You can't really do much to upgrade laptops other than upgrade the RAM, but I doubt it'd make much difference on a laptop of that age as that considered was a fairly high amount back then.
It should still work fine for older games… Both abandonware, and those that've evolved from that far back, such as:
- Second Life (online virtual world, one of the few old ones that's still alive and kicking)
- ReVolt (RC cars game… Original publishers ceased trading, but it's still being kept alive by fans)
- Star Trek Armada / Star Trek Armada II
- Star Trek: Star Fleet Command III (doesn't install too well on Windows10 machines)
- SWAT 3 / SWAT 4
- DOOM 3
- The Simpsons: Hit & Run
- Counter Strike (not sure if CS: GO will work, but might if it's a 2012? Game)… I got the full bundle via STEAM
- Half Life
- Quake 3 Arena

Of newer games that might work on it (double check specs through STEAM):
- South Park: The Stick of Truth
- Euro Truck Simulator2 may work if you turn the settings way down
- Football Manager

For more intensive newer games, you'll be better off just getting a new gaming PC as the "DOOM" (Doom4) game from 2016 apparently needs at least a minimum of 8GB of RAM + a GTX670 graphics card to have a hope in hell of working.

Detach the image panel at the hinge, along with the cables that go to power and the motherboard.
Look on Google how to change it into a small monitor.
Then build a PC box with decent components.
That is all that you can do to that laptop to make it into a "gaming pc".

The cpu in the old thing won't support modern gaming

NO. You can't change the video card since it does not have one. The Video is integrated into the motherboard. And you can't change the CPU.

Upgrading laptop hardware is… Not only near impractical, but a waste of resources. You'd need to literally start over if you want better graphics. Just build a tower.

None of what you hope to do is available to you; laptop hardware is much more restrictive to modification compare to desktops.

It is either impossible, or impractical; and either way 100% recommended NOT to do. With business grade it is likely impossible. Gaming grade or Workstation grade is impractical as in I have no desire dropping down 2x the money for 1/2 the gain while they were tricked out for gaming of their respective era.

For dated business grade I should revise my answer as impractical, they have a way out but it is still impractical.