I tried to connect 50pin external HDD/CD-RW drives to GVP's external 25pin with this cable but couldn't:
https://www.bhphotovideo.com/c/product/476681-REG/Adaptec_1816200R_DB_25_Male_to_HD.html
Assuming the cable is OK in the first place, the next thing to be painstaking and pedantic about is termination of the bus.
Speculation: GVP's card doesn't provide termination and/or termination power (it's a 5 MB/s asynchronous-only card under SCSI-1 specs, and only passive termination was prescribed at that revision level).
Fact: Active terminators (either stand-alone or found on SCSI devices) *do need* termination power to work properly
Fact: A SCSI bus must be terminated at both ends (it must be stressed that it's the bus that gets terminated at the ends, not the devices at the end. Whether, for convenience reasons, a device may also provide a terminator is a different story).
Let's begin with the external segment:
You're obviously using a SCSI box with a high-density 50pin connector, with a CD-RW and an HDD in.
Take the time to read the possible settings for the CD-RW and the HDD.
At least one of them should be able to provide both termination AND termination power.
Enable both on that device, and place it last on the box's internal cable.
Disable both on the other device and place it first on the box's internal cable.
Do not use an externally pluggable terminator on the 2nd (free) 50-pin connector.
[If you do have such an external terminator, then enable terminator power on one of the devices and disable termination for both. Cable order doesn't matter in this case.]
The other end of the bus is the internal 50pin IDC connector on the GVP card. You must either connect a device there (typically an HDD) even if it's just to enable termination on it, or use a suitable standalone IDC-plug terminator (you'll still need the short 50-pin cable for that, can't plug the terminator directly on the IDC connector as it also has a male connector).
To sum up:
1. if your setup includes devices at both ends of the bus, enable termination on them (and only them). Termination power can be sufficiently enabled on one device, although enabling it on both ends won't generally hurt anything either.
2. If your setup has no device at one end of the bus, you must use a dedicated terminator there. As long as the other end is terminated by a device with termination power enabled, it won't matter if the standalone terminator is passive or active.
3. If both ends of the bus are to be enabled by standalone terminators, then no device should have termination enabled. If the two terminators are passive, termination power is irrelevant. If however at least one of them is active, then at least one device must be configured to provide termination power (but not termination).
Of course the general SCSI guidelines still fully apply, i.e. maximum 3 meters bus length, unique SCSI IDs on the devices, parity disable if not supported by the host etc.