Create a single row gtable.

a <- rectGrob(gp = gpar(fill = "red"))
b <- circleGrob()
c <- linesGrob()
gt <- gtable_row("demo", list(a, b, c))
gt

## TableGrob (1 x 3) "demo": 3 grobs
##   z     cells name                  grob
## 1 1 (1-1,1-1) demo     rect[GRID.rect.1]
## 2 2 (1-1,2-2) demo circle[GRID.circle.2]
## 3 3 (1-1,3-3) demo   lines[GRID.lines.3]

plot(gt)

gtable_show_layout(gt)