The design consists of the features of shape, ornament, pattern and configuration of the garment adjustment strap shown in Figures 1 to 48 of the drawings. The portions shown in stippled lines do not form part of the design. FIGURE 1 is a front elevation view of the Garment Adjustment Strap in accordance with a first variant of the present design applied to a representative garment shown in stippled lines, the Garment Adjustment Strap being shown with a first configuration of an adjustment mechanism and depicted mid-adjustment, or in unfixed form; FIGURE 2 is a rear elevation view thereof; FIGURE 3 is a top plan view thereof; FIGURE 4 is a bottom plan view thereof; FIGURE 5 is a rear elevation view thereof shown in use, the Garment Adjustment Strap depicted pre-adjustment or post-adjustment, or in fixed form; FIGURE 6 is a rear perspective view thereof; FIGURE 7 is a front elevation view of the Garment Adjustment Strap in accordance with a second variant of the present design with a second configuration of an adjustment mechanism, the Garment Adjustment Strap depicted pre-adjustment or post-adjustment, or in fixed form; FIGURE 8 is a rear elevation view thereof; FIGURE 9 is a top plan view thereof; FIGURE 10 is a bottom plan view thereof; FIGURE 11 is a rear elevation view thereof shown in use; FIGURE 12 is a front perspective view thereof; FIGURE 13 is a front elevation view of the Garment Adjustment Strap in accordance with a third variant of the present design with a third configuration of an adjustment mechanism, the Garment Adjustment Strap depicted pre-adjustment or post-adjustment, or in fixed form; FIGURE 14 is a rear elevation view thereof; FIGURE 15 is a top plan view thereof; FIGURE 16 is a bottom plan view thereof; FIGURE 17 is a rear elevation view thereof shown in use; FIGURE 18 is a front perspective view thereof; FIGURE 19 is a front elevation view of the Garment Adjustment Strap in accordance with a fourth variant of the present design with a fourth configuration of an adjustment mechanism, the Garment Adjustment Strap depicted mid-adjustment, or in unfixed form; FIGURE 20 is a rear elevation view thereof; FIGURE 21 is a top plan view thereof; FIGURE 22 is a bottom plan view thereof; FIGURE 23 is a rear elevation view thereof shown in use, the Garment Adjustment Strap depicted pre-adjustment or post-adjustment, or in fixed form; FIGURE 24 is a front perspective view thereof; FIGURE 25 is a front elevation view of the Garment Adjustment Strap in accordance with a fifth variant of the present design with the first configuration of an adjustment mechanism, the Garment Adjustment Strap depicted mid-adjustment, or in unfixed form; FIGURE 26 is a rear elevation view thereof; FIGURE 27 is a top plan view thereof; FIGURE 28 is a bottom plan view thereof; FIGURE 29 is a rear elevation view thereof shown in use, the Garment Adjustment Strap depicted pre-adjustment or post-adjustment, or in fixed form; FIGURE 30 is a rear perspective view thereof; FIGURE 31 is a front elevation view of the Garment Adjustment Strap in accordance with a sixth variant of the present design with the second configuration of an adjustment mechanism, the Garment Adjustment Strap depicted pre-adjustment or post-adjustment, or in fixed form; FIGURE 32 is a rear elevation view thereof; FIGURE 33 is a top plan view thereof; FIGURE 34 is a bottom plan view thereof; FIGURE 35 is a rear elevation view thereof shown in use; FIGURE 36 is a front perspective view thereof; FIGURE 37 is a front elevation view of the Garment Adjustment Strap in accordance with a seventh variant of the present design with the third configuration of an adjustment mechanism, the Garment Adjustment Strap depicted pre-adjustment or post-adjustment, or in fixed form; FIGURE 38 is a rear elevation view thereof; FIGURE 39 is a top plan view thereof; FIGURE 40 is a bottom plan view thereof; FIGURE 41 is a rear elevation view thereof shown in use; FIGURE 42 is a front perspective view thereof; FIGURE 43 is a front elevation view of the Garment Adjustment Strap in accordance with a eighth variant of the present design with the fourth configuration of an adjustment mechanism, the Garment Adjustment Strap depicted mid-adjustment, or in unfixed form; FIGURE 44 is a rear elevation view thereof; FIGURE 45 is a top plan view thereof; FIGURE 46 is a bottom plan view thereof; FIGURE 47 is a rear elevation view thereof shown in use, the Garment Adjustment Strap depicted pre-adjustment or post-adjustment, or in fixed form; and FIGURE 48 is a front perspective view thereof.