Ūrdhvam means higher planetary system. Here it is said that api trailokya-rājyasya hetoḥ. Trailokya. Oṁ bhūr bhuvaḥ svaḥ. This is bhūr-loka. Above this, bhuvar-loka, and above that, svar-loka, heavenly planets. Oṁ bhūr bhuvaḥ svaḥ tat savitur vareṇyam. This is Gāyatrī-mantra.
So Arjuna is speaking of trailokya-rājyasya. He is ascertaining that "Even if I get the kingdom of the three worlds, what to speak of only this earthly planet, what is insignificant, even if I get the kingdom of trailokya, svarga, martya, pātāla, like that, still, I am not prepared to fight with my kinsmen, what to speak of this earthly planet." He decided like that. This is called family attachment. He is speaking, "Even in exchange of kingdom of the three worlds, I am not prepared to fight."