(wire-feed)MIG/MAG is a so-so choice for making bikes. Decent bike-appropriate tubing is fairly thin, and it'll be easy to burn a hole.
Likewise making a garden gate will be kinda borderline for how thick it can manage.
Doable, but you need to prep and plan your seams well.
A stick/arc welder would be a better choice for things like that.
If I was on a budget, and wanted to build both bikes and garden gates I'd probably opt for an oxyacetylene gas welding kit.
One set of nozzles would let you braze a bike frame, another would let you do the garden gate.
As long as you stay away from thin sheet metal, it'll do OK.