Star Alliance Gold is a great status to have because:
– it allows lounge access to all Star Alliance Gold lounges with ANY Star Alliance boarding pass
– you get one extra piece of luggage (or at least one for free) across Star Alliance
– many carriers give you a somewhat preferential treatment (exit row seats, easier re-booking when flights are cancelled etc)
Usually, Star Alliance Gold is procured with 50,000 or more miles depending on the Star Alliance member.
The easiest way to attain the status, however, has long been Aegean Airlines with just 20,000 miles. It requires just 19,000 credited miles (as you'll get your first 1,000 miles just by signing up through this link).
If you credit at least 3,000 miles you will become Miles & Bonus Blue or Star Alliance Silver. From that day you have 12 months to collect the remaining 16,000 miles to attain Gold status.
Aegean Airlines has its Earnings Table here. You will need to know your booking class, which is usually the first letter of your fare code. Almost all OTAs make this visible at some point in the booking process but it can be tricky to find out. ITA Matrix usually makes it very visible.
If you have a flight coming up with, say, Air Canada business class to Asia (as a paid ticket) you just need about 8,000 miles flown to reach the threshold since you will earn a 100% bonus. The same is true for many other Star Alliance carriers, but do check it carefully – Lufthansa has a business class paid ticket bucket that does NOT earn any miles.
Most economy class tickets (like United's cheap K fares or Turkish cheap W fares) won't earn 100% miles. Some other carriers like Ethiopian Airlines give you 100% credit. One flight with Ethiopian from New York to South Africa is enough to earn you Star Alliance Gold. Such fares can often be had for just about $1,000.
Another true and tested way is US Airways Transcontinental domestic flights. Although US Airways exited Star Alliance, it kept a number of profit-making partnerships as well as the agreement with Aegean and it still earns 100% miles on Aegean.
US Airways flights from coast to coast can be had for $250 if booked in advance. You will need to complete few more than 3 round-trips for your successful status challenge.
– Do I 'lose' the miles credit to Aegean?
Aegean has its own rules for award redemptions but allows one-way award flights for 12,500 each. The Award Chart can be found here.
– Will the Greek airline be around much longer?
Ever since the collapse of the Greek economy this question has been raised, but the airline has battled through. I've flown it several times myself and it seems modern and very efficient. Think of LAN International but in Southern Europe.