Macbook Pro crashes when G-Drive is plugged into Thunderbolt 3

Disconnect all other external devices in case one of them is causing a problem.

Make sure to connect the drive directly to the computer.

Try each of the other USB-C ports especially on the other side of the laptop. You can also try rotating the USB-C connector 180º upside down as well.....this can sometimes work if the port is half bad.

Make sure to test with another cable as well. If the drive has more than one USB-C port, then try using the other one assuming it is equal to the first one in abilities.

The I/O Boards (aka USB-C ports on the laptop) are a prime candidate for issues because on the 2016-2017 models the connectors wear causing loose connections. Plus in my own personal experience supporting my organization's Macs, the USB-C ports seem to attract liquids & have liquid damage.

The Logic Board is another possibility. Since USB-C ports handle both data transfers and support supplying power to the laptop, they tend to have a higher rate of failure due to the chips managing the circuitry on those ports. Sometimes only one port or chip is damaged/bad, but it can sometimes affect each of the other ports even on the other side of the laptop.

If you have the two port non-touchbar model, then you are in trouble because the USB-C ports are integrated onto the Logic Board, so when they become worn & loose, or damaged, then it is time to replace the laptop since the Logic Board is expensive.

Besides, I do not recommend anyone repairing any of the 2016-2020 Intel Macs for multiple reasons since they have so many design & manufacturing issues/flaws as well as becoming outdated very quickly. Money is better put towards a new laptop.

You have a 90 day part/repair warranty depending on how the repair was performed. Take the laptop back to have them re-examine it. Perhaps an I/O Board connection is not fully seated. Make sure to take the external drive and what ever other easily portable items that are having problems as well as one which is working fine so the tech can see the issue first hand.

In order to replace the Top Case/Battery Assembly an AASP has to remove every component, so there is always a chance something went wrong somewhere.

Was your macOS version upgraded while in for service?

the Drivers for G-drive for recent MacOS require two specific add-ons for proper operation.

One is a divergent version of the Promise extensions (NOT the default version being carried along in MacOS).. The other is a proprietary extension for the drive. See the drive-maker's web site for the full story.

"crashes on first access" is a serious problem that is very difficult to deal with.

perhaps if you Restart in Safe mode or Recovery mode you could run a disk Utility Repair Disk from one of those places.

By far the easiest way to cause poor performance, instability, overheating and crashing is to install ANY third-party speeder-uppers, Cleaners, Optimizers, or Virus scanners, Bit Torrent, or a VPN that you installed yourself.

¿Are you running anything like that?