Anchoring a metal gazebo to concrete is straightforward, although you will need a masonry drill bit and concrete fasteners. It is extremely important that you lay out the base for the structure according to the manufacturer's instructions and position the baseplates where indicated along the layout.
Follow the gazebo manufacturer's instructions and lay out the base for the gazebo exactly where you want it positioned on the concrete slab. Measure to the corners carefully and use a framing square and snapped chalk lines to make sure the layout is perfectly square.
Make marks perpendicular to the chalk line where each of the baseplates for the gazebo supports is to be fastened to the concrete.
Take one of the baseplates and transfer its outline and the position of all the holes to a flat piece of cardboard or poster board to make a bolt pattern template. Drill or poke a small hole in the centre of each bolt mark so the tip of your marker will fit through easily.
Use the template to mark the position of the holes for each baseplate on the concrete slab layout. Use a colour of permanent marker you can see easily.
Put the masonry bit in your drill. Wrap a piece of tape at the correct height on the drill bit to provide a depth gauge. The depth should be enough so the concrete slab will receive the concrete expansion fasteners but no deeper than the holes need to be. Keeping the drill perpendicular to the concrete slab, drill the holes you marked for the bases.
Position the baseplates over the drilled holes and slip the fasteners down through them and into the holes. Make sure they are seated all the way down in the holes and tap them in place if needed. The holes in the bases will determine the diameter of the fasteners. Your hardware or fastener store can provide you with the correct length to resist the shear and tension stresses that the gazebo will put on the fasteners.
Tighten the fasteners slowly so that they expand and grip the sides of the holes. You can use your drill and driver for this if it has a torque setting, but it is better to do this by hand with a socket and wrench. If you overtighten a fastener you may break it, have to reposition the base, drill a complete new set of holes and replace the fasteners you already have in place.
Assemble the gazebo working up from the secured baseplates by fitting together the upright structural members and canopy as directed in the instructions.
Draw your concrete slab to scale or use graph paper first to position the structure where you want, then transfer the measurements to the actual slab. A carbide or diamond-tipped masonry bit will reduce the time it takes to drill the holes.
Be sure and buy quality and medium to heavy duty rated fasteners from a reputable hardware or fastener store. Many sold in large home improvement stores are substandard. Even if your gazebo was shipped with fasteners many manufacturers skimp on those they ship with kits so buy new ones you can trust.