"2G" is a general fit, as opposed to "1G" and "3G" for tight and loose fit between box and pin.
Yes, the 2G is applied to the joint, but is specified as "2A" for the pin or screw, "2B" for the box or nut. You would call out 1.750-6 ACME-2A for a 6 tpi pin and 1.750-6 ACME-2B for the corresponding box.
The tolerance as well as thread allowance is governed by the classification of thread. As mentioned, Machinist Handbook is a very good place to start.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada