what is the major advantage that gp2 posses on comparison with gp3

The major advantage of gp2 over gp3 is that IOPS increases automatically as you increase the size of the volume. you don't have to set IOPS yourself, gp2 adjusts it based on the volume size, which is easier for simple setups.
Another advantage of gp2 is that it's been around longer and is well supported in older AWS setups. It's a safer choice for existing setups & older instance types.

Whether throughput gives more performance than IOPS? If so is there min limit to use throughput

Throughput is better than IOPS for tasks that deal with large amount of data like big data processing. IOPS is better for tasks that need fast access to small files like databases. To use gp3 volumes for high throughput, the min size is 1GiB & for st1 volumes, the min limit is 500 GiB. You can increase throughput by changing the volume size or IOPS, but its most useful for tasks that need high data transfer speeds.

What is the risk of detaching a volume from an instance while it's in use?

. Data Loss or Corruption
Peding files may be lost(currently getting written)
File systems may become corrupted.

Filesystem Inconsistency

what are the steps involved to modify the IOPS or Size of the volume?

Whether throughput gives more performance than IOPS? If so is there min limit to use throughput