Rahu and a Gemini Lagna: Mercury Is the Dispositor Only in Gemini

For a Gemini lagna, the lagna lord is Mercury, and Mercury also rules the 4th. Rahu does not become a functional benefic because the rising sign is Mithuna. Rahu takes Mercury's nature only when Rahu occupies Gemini or Virgo, because only then is Mercury the dispositor. Rahu in the Gemini lagna itself is Rahu in the 1st, disposited by the lagna lord. That is a specific and often ambitious, unusual self. Rahu in the 10th, which is Pisces for this lagna, is disposited by Jupiter, who rules the 7th and the 10th. That Rahu is about marriage and career, and it is Jupiterian, not a gift from the Gemini label.

Mercury is a conditional benefic. Alone or with benefics it behaves well. With malefics it picks up their nature. Rahu sitting with Mercury can therefore spoil the very dispositor it depends on. A Gemini lagna with Rahu in the 11th, Aries, answers to Mars, who rules the 6th and the 11th for this lagna. Gains and competition, not a gentle beneficence, are the topics. If the question meant Gemini Moon rather than Gemini lagna, the houses are unknown until a birth time exists. A Moon sign cannot declare Rahu benefic. It can only host Rahu if Rahu is transiting or natally in that sign.

Rahu in the 3rd, 6th, 10th, or 11th from the Gemini lagna is the upachaya pattern tradition prefers. From Gemini those houses are Leo, Scorpio, Pisces, and Aries, ruled by the Sun, Mars, Jupiter, and Mars. The dispositor changes in each sign. Rahu in the 7th, Sagittarius, is disposited by Jupiter as 7th lord and is a marriage placement, not an automatic benefit. Eighteen years of Rahu mahadasha will tell whichever of these is natal. Do not spend those years believing Gemini saved you, and do not spend them believing Rahu is poison. Name the sign Rahu is in. The lord of that sign is your answer.