To take a full profit from our modern warfare knowledge, Gary would need:
- knowledge of the warfare theories (Sun Tzu and Clausewitz, for example),
- knowledge of past battles and used tactics (review of what was done, why, and what was the outcome would be profitable),
- knowledge of the technology/tactics and troops available to him (nice and great to know about aerial coverage, but well, not going to be too useful for him),
- capacity to adapt to his environment (which requires the knowledge mentioned earlier).
And now, assuming he has all the preceding points, here are a couple of things he could/should be doing...
... as a Lord
The first thing it to make sure that the war/battle is the best alternative. The army is a tool for politics. But not the only one. He should define precisely the goals he wants to achieve, and evaluate the various means to achieve it. It the war is the best option, the scope and objective should be clearly defined (see the USA in Irak, Sun-Tzu, Clausewitz).
The second thing is that no one can wage a war without money. So he needs to boost his economy, increase his surplus, find financial support, etc. Once this has been achieved the war can be considered (e.g. Charles VII in the 100-years war).
The third element is to make sure that he has a solid weapon industry or supply. He needs to arm his troops.
For longer-term, he needs to make sure that he stays on top of things. So universities and schools, might be able to help with innovative strategies/tactics/weapons.
Another point of importance, is that he is well read with Macchiavel. He knows how to rule his own Lordship as well as dominions he might get in possession of.
... as the Army's Chief-General
The Army is a tool that should be ready for the politics. They should prepare to various possibilities, but only act upon the decision of the politics.
As Sun Tzu teaches us, the first thing is to know you enemy. The Chief-General should organise spies networks in order to gather a good information on the armies of any potential adversaries.
Then, modern and current armies rely essentially on well-trained professional troops. He needs to recruit and train people. Note that one possible advantage of really modern knowledge: women can fight equally well, they could increase the size of the army. But that might be some cultural shock. But maintaining a professional army during peace is extremely expensive, so he needs to find methods to do it. Like regular short trainings (see Swiss' military service).
He also needs to invests considerably in logistics. Food, money and anmunitions should flow without issue during a campaign. As mentioned in other answers, hygiene is also a crucial element.
Then military engineering might play a crucial role. He should ensure to have a supply of engineers who could provide technical solutions (crossing a river, take a castle, etc.).
Even if he takes on the three posts described here, he can't be everywhere, every time. Furthermore, recent strategies used fake movement and armies to mask the real intentions (see Operation Fortitude). So he needs to have a set of trusted, capable generals to carry-out the order and be ready to adapt to the situation. This can be done by knowing the people, and promote worthy ones to posts with responsibilities, regardless of their origin (note that this may not be so well accepted for a brutal change). But this also needs to have training... and trainers. Which would rely on the universities and schools developed as mentioned.
... as a Field General
Here, the knowledge of the past battles would be very important.
Scouts are important to know the preparation of the enemies as well as recognise the terrain.
He knows to separate the strategy from the tactics. A battle may be lost, if it gives an advantage for the campaign.
Find the best field. Always go to battle when the field favour his army. Or at least does not go against him. Terrain knowledge from the scout is crucial.
Use a combination of the various arms available. Typically light and heavy cavalries, peakesmen, archery for European's MA. Use smaller specialised units and deploy them to counter the opponents tactics. Keep reserve troops.
Two very important lessons to take: mobility. Even the best troops (hoplites) could be out-maneuvered. He should make sure that his troops are reactive and can be quick to move. And communication: the orders and reports should rely on well trained troops and officers, as well as well established communication channels. The next tactical move should be communicated effectively to the soldiers.
As he prepared his own troops to be well supplied, mobile and well informed, he knows the crucial importance of those. He may then attempt to disrupt his enemies' supply or communication lines.
Assuming that apart from the knowledge, he has the required capacities, he might greatly profit from that obtained knowledge in his military actions. However, many/some of those need time to be effective. In our history, some of those elements took really centuries to get some effective value.