Modern warfare requires modern weapons/tools. So... many tactics, formations and most rules do not apply because neither enemy nor they themselves do have the tools to make them useful.
What would come in handy would be the modern training methods, ranking system, execution of commands/discipline and how supply chains are set up. Also, many 'modern' ideas, like hygiene and that alcohol should not be a replacement of water.
Raising a professional army would already be quite a benefit, apply what we now know about health and training and it would be quite the formidable army too. Battle in these times usually took place at the battlefield, literally. Two opposing armies would meet at a specific area and fight to the last man standing.
Using what would probably be considered underhanded methods Gary could probably overthrow many kingdoms with guerilla warfare, spies/infiltration, assassination and less effective versions of the Blitzkrieg tactic (conquering as fast as possible, really effective if you have machines to get around, tanks etc). Also preparing the country for war. Setting up supply lines, shelters for wounded soldiers etc. using propaganda to get the population riled up and change their mindset to pro-war. Also training most soldiers of the professional army to lead untrained people. If it were required stocking up on manpower by drafting from the general population would work quite well too.