Gaining

One immediate thing would be to GET used to eating super big meals. You're nearly 6', and depending on how long you've been on hormones (if at all) you've still got the metabolism of a fit young man, so to overwhelm it with sheer caloric intake will take a lot of bloody calories. You don't need to stuff yourself to bursting point with every meal; just go a little bit past full every day or so, so that it takes a bit more food to get full the next time.

Another option is to start fasting for up to a day at a time and then eating all you want. This is to try and shift your metabolism into starvation mode, i.e. "Got to store as much fat as I can because I don't know when the next meal will be."

For a transgender, the timing of a gain is critical because it determines where the fat will go. If you've been on hormones for long enough, and especially if you're post-op, you'll tend to gain the weight in the right places for a female (hips, butt, thighs and so on). If not, you'll probably still gain weight like a male, and a bigger belly by itself won't look feminine compared to the alternatives.

If you don't think you're at the right stage yet, maybe hold off on gaining and just switch any resistance training to cardio, to get lean rather than muscular.

Whatever you do, best of luck.

Eating as much as you can through the day, have that all important bedtime snack that's very high in saturated fat

The other thing, though some might not want to try them, is weight gain shakes, if you are generally sedentary they can help you really pile on serious weight in addition to a very fattening diet, one big tub I got in 2010 helped me boost my at the time 47" waist to 53.5" in about 7 to 8 weeks.

Eat lots of desserts, plenty fried foods, and don't resist the takeaway on the way home.

If your metabolism is fast you will find it hard to gain weight fast.