newcolumntype{C}[1]{>{centeringarraybackslash}p{#1}}
newcolumntype{L}[1]{>{raggedrightarraybackslash}p{#1}}
.
.
begin{table}[!ht]
centering
caption{Average error metrics.}
smallskip
begin{tabularx}{columnwidth}{L{1cm} C{1.5cm}C{1.5cm}C{1.5cm}  C{0.01cm} C{1.5cm}C{1.5cm}C{1.5cm}}
toprule
& multicolumn{3}{c}{UHFUS} & & multicolumn{3}{c}{HFUS} \
cmidrule(){2-4} cmidrule(){6-8}
Metric & {G1} & {G2} & {G3} & & {G1} & {G2} & {G3} \
midrule
D & 0.91$pm$0.01 & 0.90$pm$0.01 & 0.89$pm$0.01 & & 0.91$pm$0.01 & 0.91$pm$0.02 & 0.91$pm$0.02 \
H ($mm$) & 0.0973$pm$0.019 & 0.0917$pm$0.019 & 0.105$pm$0.02 & & 0.292$pm$0.023 & 0.281$pm$0.065 & 0.273$pm$0.04\
DFP & 1.35 & 5 & 4.93 & & 12.3 & 5 & 34.6 \
DFN & 1.35 & 5 & 4.93 & & 12.3 & 5 & 34.6\
bottomrule
end{tabularx}
label{table1}
end{table}