def next_available_row(worksheet):
    str_list = list(filter(None, worksheet.col_values(1)))
    return str(len(str_list)+1)

scope = ['https://spreadsheets.google.com/feeds']
credentials = ServiceAccountCredentials.from_json_keyfile_name('auth.json', scope)
gc = gspread.authorize(credentials)
worksheet = gc.open("sheet name").sheet1
next_row = next_available_row(worksheet)

#insert on the next available row

worksheet.update_acell("A{}".format(next_row), somevar)
worksheet.update_acell("B{}".format(next_row), somevar2)