Summary:
ר"ג maintained (like ר"ע) that initially מעשר was to be given specifically only to the לויים. The קנס of עזרא was to give it to the כהנים as well. The כהנים would (watch and) close the doors in three places in the ביהמ"ק; the other doors were (watched and) closed by the לויים.
And were they standing beside the field of רבן גמליאל, etc. - וכי בצד שדהו של רבן גמליאל היו עומדין
Summary:
To be considered משתמרת a חצר needs to be משתמר לדעת הקונה. If one has a property adjacent to a חצר, it is considered as if the owner is standing beside the חצר.