What does address like 0xABCD_EFGH means in a memory map?
I would interpret "(GPDI0_3–GPDI120_123)" as referring to 31 groups of signals or registers: the first being GPDI0 through GPDI3, the second being GPDI4 through GPDI7, the third GPDI8-GPDI11, etc. up to the last two being GPDI116-119 and GPDI120-GPDI123.  Note that that would be 124 signals total, not 123.

In a 32-bit device, it would be common to have groups of four single-byte registers which could either be accessed individually as bytes, or as a group (32-bit word).  The manufacturer probably suggests GPDI0_3 as a symbolic name for the 32-bit word comprising the first four registers, GPDI4_7 for the 32-bit word the next four, etc.