Techncially, by lore descriptions in 5e, Halflings are slightly shorter by averages than Gnomes, but Gnomes tend to have more pronounced physical features owing to their traces of fey lineages.
In other media and other sources that aren't D&D, gnomes are made smaller than halflings, but that's not the case in this particular world space. The difference is only very minor - Halflings average 'just under 3 feet', while gnomes average 'at 3 feet'
(You might also be variously pleased or disappointed to know that modern D&D halflings don't have the hairy feet any more either, as part of their efforts to shift away form and differentiate from Tolkien hobbits)