Join dataframes... So simple but I can't work it out!

[snakes]

Given it was only 14 rows long I just added the index numbers to the line converting the array to a dataframe...

# Convert to dataframe
gender_predictions = pd.DataFrame(predictions, columns=['gender'], index=[13,14,48,50,55,61,71,73,83,84,103,106,149,162])

My next issue (!)...
Join/merge/concat the two dataframes. I'm going round in circles chasing for an answer I don't know exists!

I just want to 'merge' nans_train and nans_test_tr so that the index/ID is sorted in order. Here is a section that shows 13 and 14 missing...

Output:
nans_train
ID	age	gender	usage_meeting_place	usage_worship	usage_arts	usage_wellbeing	usage_connections	usage_model_sustainability	usage_flexible_community/church	usage_services	usage_festivals	usage_reflection	modify_exterior	modify_interior	modify_sustainable_building	modify_layout	modify_cafe
0	3	1	2.0	3	3	2	1	3	4	4	2	3	1	3	4	2	3	1
1	4	2	1.0	4	4	4	4	4	3	4	4	4	3	4	4	4	4	4
2	5	2	1.0	4	3	4	4	4	4	4	4	4	3	4	4	4	4	4
3	6	2	2.0	4	2	4	4	4	4	4	2	2	3	2	3	4	4	4
4	7	2	1.0	4	3	4	1	1	0	4	2	2	2	4	4	4	4	0
5	8	2	2.0	4	4	4	4	4	4	4	4	4	4	4	4	4	4	4
6	9	1	2.0	3	2	3	3	3	3	2	2	1	1	3	3	4	3	3
7	10	1	2.0	4	2	2	3	4	2	2	2	2	4	3	2	2	2	4
8	11	1	1.0	4	0	3	3	4	3	2	2	3	2	3	4	3	4	4
9	12	3	1.0	4	4	4	4	4	4	4	4	4	4	3	3	3	4	4
10	13	2	1.0	4	4	4	3	3	3	3	3	4	2	4	4	4	4	4
11	14	3	1.0	4	4	4	4	4	4	4	4	4	4	2	3	3	2	3
12	15	2	2.0	4	3	3	4	4	4	4	3	3	3	3	4	4	3	4
15	18	2	2.0	4	4	4	4	4	4	

Output:
nans_test_tr
ID	age	usage_meeting_place	usage_worship	usage_arts	usage_wellbeing	usage_connections	usage_model_sustainability	usage_flexible_community/church	usage_services	usage_festivals	usage_reflection	modify_exterior	modify_interior	modify_sustainable_building	modify_layout	modify_cafe	gender
13	16	2	4	2	4	4	4	2	2	2	2	2	3	3	2	4	4	2.0
14	17	1	4	4	4	4	4	1	4	1	1	0	2	3	4	3	0	1.0