Is there a word or phrase that encapsulates this recurrent scenario? Let me give a couple examples.
A limited amount of land is cheap or free in a given area. The people who get there first get all the land. They can then set the price so that people who come later and still want land might end up paying much much more. This applies to any limited resource that is desirable.
On sister site Stackoverflow people ask programming questions. The first people who asked and answered the most basic questions about given languages in the early stages of the site received thousands of upvotes and reputation points. Basic questions are broadly applicable to learners of a programming language, so whoever was there first is by far the most visible and rewarded. Those questions can never be asked again (in theory) without being marked as duplicates.
"Early adopters" comes to mind, but it's limited in scope and only partially applicable as a category of people. The phrase doesn't imply that there are definite, intrinsic, and limited benefits to gain by being among the first to do something, benefits which will not be available, or available in diminishing amounts, to later arrivals because they will have already been used up or claimed.
Best Answer
The idiom is "The early bird catches the worm."
"The early bird catcheth the worm." first recorded in John Ray's A collection of English proverbs 1670, 1678.
This has been shortened to "early bird" and "early riser" but it is a bit towards the colloquial and not exactly a technical term.
"Early adopter" has no value attached to it. You could be an early adopter and not benefit or gain anything (even worse, gain pain) from it.
Early bird has the problem of context, where it could just mean that you were someone who does things earlier than normal: an early bird special is for people who eat dinner before a dinner rush, for example, and not wholly about "benefiting from early adoption."
Long story short, there is no single term for "early adopter + one who gained from adopting early" aside from the idiom.