If you are born with the Gemini ascendant and Mercury is placed in the 12th house, then this placement has significant impact on your life. Here, Mercury is in its own sign state and considered to be in a friendly position.
Let us first understand what it means when Mercury rules your 1st and 4th houses. Mercury is the planet of communication, intellect, and reasoning capabilities. Since it holds royalty over ascendant such people have a natural proclivity towards communication skills. They are articulate speakers and very expressive in their body language as well. They may appear anxiety prone due to their innate urge to have quick results at every stage of life but they also develop out of the box solutions quickly thus giving them an edge over competition.
Fourth house represents mother, home and vehicles for this native since mercury rules it. The individual will have a deep connection with their mother who would impart them exceptional communicative skills. Home environment would be full of lively debates and intellectual talks which would create an intellectual passion within the native for life long pursuits.
Coming back to Mercury placed in the 12th house, which signifies one's innermost feelings, imagination power that keeps them awake at night along with their sub-conscience mind that acts as an untapped resource often left unexplored by most human beings. This placement makes an individual very intuitive where they start picking up information from unseen sources which if used properly could reveal great opportunity elements otherwise hidden from plain sight.
This placement can make you feel like you are on some sort of mission- important ideas keep flooding your thought process and everything feels super important yet impossible to bring into fruition without detailed investigation steps taken towards each respective idea considering all perspectives possible.
The aspect of Mercury onto the sixth house creates a formidable combination where extreme productivity drives result-oriented actions throughout one's workday which faciliates intense cerebral activity yielding deep problem-solving acumen for tasks related at workplace. One develops an almost unusual talent to think outside the box leaving their peers & competitors baffled always.
As Mercury also happens to be a natural karaka of the 10th house, it seeks intellectual power in professional zone. An individual will embrace value-based innovative technologies which change norms across organizations making them very adept at manipulating loopholes in system structure.
This placement is noted for producing an intense desire for travel or wandering into foreign lands - preference towards eastern countries mostly. This wanderlust comes from Mercury's affinity towards Ninth House which rules long-distance travel & philosophy of life where one is either traveling mentally or physically with the goal of seeking knowledge from various sources which would put them in good stead going forward.
In conclusion, placements such as Mercury positioned in Gemini Lagna's 12th house can have significant impact on a person's life affecting areas like communication skills, innermost feelings, sub-conscious mind, and even wanderlust to foreign lands. It is important to note that while astrology can offer insights and guidance, it should never be used as a tool for deciding important decisions in your life. You are still ultimately responsible for your own choices and actions.