The ornamental design for a stringactiongauge for string musical instrument, as shown and described.
FIG. 1 is a top perspective view of a string action gauge for string musical instrument showing my new design;
FIG. 2 is a bottom perspective view thereof;
FIG. 3 is a top elevation view thereof;
FIG. 4 is a bottom elevation view thereof;
FIG. 5 is a right edge view thereof;
FIG. 6 is left edge view thereof;
FIG. 7 is a front edge view thereof; and,
FIG. 8 is a rear edge view thereof.
The broken lines in FIGS. 1-8 represent the portions of the string action gauge for string musical instrument which form no part of the claimed design.