The yellow gun isn't necessary. You just need some planning beforehand.
With the yellow gun, you can move a bridge across.
However, it is solvable with the blue gun. Fall down, pick up cubes from HMM, make a staircase downwards until a safe height below disintegrator field, then tower up until that height until you reach your own staircase. Bridge over if necessary, and climb to the other side.
There's a space.
Using the red gun, first, collect cubes from HMM if you have none. Make more cubes from that. Then start filling out a floor from the space below you where the floor recedes slightly downward. Walk across your expansion bridge. If you're speedrunning, make the bridge go partway, then jump the gap.
This problem is solvable with the yellow gun. Make bridge with 'WTF!?' just same as 'HMM', but it will be more difficult.