Just do this small trick.
Remove fxLayoutGap and add a margin to the child element and add the minus value of that same margin to the parent element.
<div fxLayout="row wrap" style="margin-right: -16px;">
<div fxFlex="calc(50% - 16px)" style="margin-right: 16px;">
</div>
<div fxFlex="calc(50% - 16px)" style="margin-right: 16px;">
</div>
</div>
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