To perform this, know the pneumonic for PLL locations first.
LRC (Lower Right Corner),
LLC (Lower Left Corner),
ULC (Upper Left Corner),
URC (Upper Right Corner)
Then after knowing the GPLL locations pneumonic, use the "ldc_set_location" constraint.
Example: The desire GPLL location is to use lower right corner GPLL.
Answer: ldc_set_location -site {PLL_LRC} [ get_cells {pll_wb/lscc_pll_inst/gen_no_refclk_mon.u_PLL.PLL_inst} ]
Also, check the software generated constraint at Place & Route Reports > Signal/Pad > #PLL for the template.