I resolved this!
Turns out that the layout works properly if I use layout instead of gridplot.
Original implementation commented out, and working implementation shown below:
#grid = gridplot(
#children = [
#[hor_layout_date_slider],
#[hor_layout_time_cb],
#[scatter_fig, vert_layout_1, vert_layout_2],
#[data_table]
#],
#toolbar_options=dict(logo=None),
#sizing_mode='fixed',
#)
grid = layout(
children=[
[hor_layout_date_slider],
[hor_layout_time_cb],
[scatter_fig, vert_layout_1, vert_layout_2],
[data_table]
],
sizing_mode='fixed',
)