Driving props is a pretty simple thing to do in GMod. You can find extra addons to do this, but you can also do it without any mods or addons!
Steps
Method 1
Combining a Drivable and Non-Drivable Vehicle Using "No Collide"
Google images Step 1 Go to the Spawn Menu by pressing Q.Go to the Spawn Menu by pressing Q. Then select Half-Life 2 > Vehicles (or any other tab with vehicle props), and spawn it.
Google images Step 2 Spawn a drivable vehicle by going to the Spawn Menu > Vehicles.Spawn a drivable vehicle by going to the Spawn Menu > Vehicles. Then spawn a car of any kind (i.g. TDM cars).
Google images Step 3 Go to the Tools tab in the Spawn menu.Go to the Tools tab in the Spawn menu. When in the Tools tab, select the No-Collide Tool.
Google images Step 4 Using No-Collide, right-click on the prop.Using No-Collide, right-click on the prop. Then click on the drivable car. That should combine them without any problems.
Google images Step 5 Select the Physics Gun and put the prop car inside the drivable car.Select the Physics Gun and put the prop car inside the drivable car. Make sure that both driver's seats are aligned.
Google images Step 6 Use the Weld Tool and weld the 2 cars together (Spawn Menu > Tools > Weld).Use the Weld Tool and weld the 2 cars together (Spawn Menu > Tools > Weld).
Google images Step 7 Use the Color Tool (Spawn Menu > Tools > Color) and make the drivable car 100% transparent.Use the Color Tool (Spawn Menu > Tools > Color) and make the drivable car 100% transparent.
Method 2
Using an Addon
Google images Step 1 Press ⇧ Shift+Tab ↹ to open the Steam overlay.Press ⇧ Shift+Tab ↹ to open the Steam overlay. Then click on the Web Browser button.
Google images Step 2 Go to Pause Menu > Addons > Open Workshop.Go to Pause Menu > Addons > Open Workshop.
Google images Step 3 Use the Search Bar to search for addons that might help you out.Use the Search Bar to search for addons that might help you out. Try searching "Driveable Props Tool" or similar searches like it.