It used to be free so maybe you can find an old version and try it out. There was also another mod which IIRC came out before Garry's did but the guy stopped developing it, called Jay mod or something, I forget.
You can attach propulsion units to things. At one point I remember making a radio-controlled boat which was quite fun. I haven't played it for a long time so it's probably changed and expanded a lot since then.